diff --git a/diagnose.cabal b/diagnose.cabal
index 2137f7d..6f7647d 100644
--- a/diagnose.cabal
+++ b/diagnose.cabal
@@ -1,6 +1,6 @@
 cabal-version: 1.12
 
--- This file has been generated from package.yaml by hpack version 0.34.6.
+-- This file has been generated from package.yaml by hpack version 0.34.4.
 --
 -- see: https://github.com/sol/hpack
 
@@ -65,8 +65,10 @@ library
       BlockArguments
   ghc-options: -Wall -Wextra
   build-depends:
-      base >=4.7 && <5
+      array ==0.5.*
+    , base >=4.7 && <5
     , data-default >=0.7 && <1
+    , dlist ==1.0.*
     , hashable >=1.3 && <2
     , prettyprinter >=1.7.0 && <2
     , prettyprinter-ansi-terminal >=1.1.0 && <2
@@ -108,9 +110,11 @@ test-suite diagnose-megaparsec-tests
       BlockArguments
   ghc-options: -Wall -Wextra -threaded -rtsopts -with-rtsopts=-N
   build-depends:
-      base >=4.7 && <5
+      array ==0.5.*
+    , base >=4.7 && <5
     , data-default >=0.7 && <1
     , diagnose
+    , dlist ==1.0.*
     , hashable >=1.3 && <2
     , prettyprinter >=1.7.0 && <2
     , prettyprinter-ansi-terminal >=1.1.0 && <2
@@ -147,9 +151,11 @@ test-suite diagnose-parsec-tests
       BlockArguments
   ghc-options: -Wall -Wextra -threaded -rtsopts -with-rtsopts=-N
   build-depends:
-      base >=4.7 && <5
+      array ==0.5.*
+    , base >=4.7 && <5
     , data-default >=0.7 && <1
     , diagnose
+    , dlist ==1.0.*
     , hashable >=1.3 && <2
     , prettyprinter >=1.7.0 && <2
     , prettyprinter-ansi-terminal >=1.1.0 && <2
@@ -185,9 +191,11 @@ test-suite diagnose-rendering-tests
       BlockArguments
   ghc-options: -Wall -Wextra -threaded -rtsopts -with-rtsopts=-N
   build-depends:
-      base >=4.7 && <5
+      array ==0.5.*
+    , base >=4.7 && <5
     , data-default >=0.7 && <1
     , diagnose
+    , dlist ==1.0.*
     , hashable >=1.3 && <2
     , prettyprinter >=1.7.0 && <2
     , prettyprinter-ansi-terminal >=1.1.0 && <2
diff --git a/package.yaml b/package.yaml
index 512dff3..8546254 100644
--- a/package.yaml
+++ b/package.yaml
@@ -8,11 +8,13 @@ category: "Error Reporting"
 
 dependencies:
 - base >= 4.7 && < 5
+- array >= 0.5 && < 0.6
+- data-default >= 0.7 && < 1
+- dlist >= 1.0 && < 1.1
+- hashable >= 1.3 && < 2
 - prettyprinter >= 1.7.0 && < 2
 - prettyprinter-ansi-terminal >= 1.1.0 && < 2
 - unordered-containers >= 0.2 && < 0.3
-- hashable >= 1.3 && < 2
-- data-default >= 0.7 && < 1
 - wcwidth >= 0.0.1 && <1
 - text >= 1.0.0.0 && <= 2.0
 # ^^^ This is unfortunately required, but as 'prettyprinter' already depends on it, it will already have been fetched
diff --git a/src/Error/Diagnose/Diagnostic/Internal.hs b/src/Error/Diagnose/Diagnostic/Internal.hs
index 8a58a07..d4f6cac 100644
--- a/src/Error/Diagnose/Diagnostic/Internal.hs
+++ b/src/Error/Diagnose/Diagnostic/Internal.hs
@@ -20,13 +20,16 @@ import Control.Monad.IO.Class (MonadIO, liftIO)
 import Data.Aeson (ToJSON(..), encode, object, (.=))
 import Data.ByteString.Lazy (ByteString)
 #endif
+
+import Data.Array (listArray)
+import Data.DList (DList)
+import qualified Data.DList as DL
 import Data.Default (Default, def)
-import Data.Foldable (fold)
-import Data.HashMap.Lazy (HashMap)
+import Data.Foldable (fold, toList)
 import qualified Data.HashMap.Lazy as HashMap
 import Data.List (intersperse)
 import Error.Diagnose.Report (Report)
-import Error.Diagnose.Report.Internal (prettyReport)
+import Error.Diagnose.Report.Internal (FileMap, prettyReport)
 import Error.Diagnose.Style (Annotation, Style)
 import Prettyprinter (Doc, Pretty, hardline, unAnnotate)
 import Prettyprinter.Render.Terminal (hPutDoc)
@@ -38,11 +41,11 @@ import System.IO (Handle)
 --   to create a new empty diagnostic, and 'addFile' and 'addReport' to alter its internal state.
 data Diagnostic msg
   = Diagnostic
-      [Report msg]
+      (DList (Report msg))
       -- ^ All the reports contained in a diagnostic.
       --
       --   Reports are output one by one, without connections in between.
-      (HashMap FilePath [String])
+      !FileMap
       -- ^ A map associating files with their content as lists of lines.
 
 instance Default (Diagnostic msg) where
@@ -54,7 +57,7 @@ instance Semigroup (Diagnostic msg) where
 #ifdef USE_AESON
 instance ToJSON msg => ToJSON (Diagnostic msg) where
   toJSON (Diagnostic reports files) =
-    object [ "files" .= fmap toJSONFile (HashMap.toList files)
+    object [ "files" .= fmap toJSONFile (fmap toList <$> (HashMap.toList files))
            , "reports" .= reports
            ]
     where
@@ -85,7 +88,7 @@ prettyDiagnostic ::
   Diagnostic msg ->
   Doc Annotation
 prettyDiagnostic withUnicode tabSize (Diagnostic reports file) =
-  fold . intersperse hardline $ prettyReport file withUnicode tabSize <$> reports
+  fold . intersperse hardline $ prettyReport file withUnicode tabSize <$> toList reports
 {-# INLINE prettyDiagnostic #-}
 
 -- | Prints a 'Diagnostic' onto a specific 'Handle'.
@@ -117,7 +120,10 @@ addFile ::
   String ->
   Diagnostic msg
 addFile (Diagnostic reports files) path content =
-  Diagnostic reports (HashMap.insert path (lines content) files)
+  let fileLines = lines content
+      lineCount = length fileLines
+      lineArray = listArray (0, lineCount - 1) fileLines
+   in Diagnostic reports (HashMap.insert path lineArray files)
 {-# INLINE addFile #-}
 
 -- | Inserts a new report into a diagnostic.
@@ -127,7 +133,7 @@ addReport ::
   Report msg ->
   Diagnostic msg
 addReport (Diagnostic reports files) report =
-  Diagnostic (reports <> [report]) files
+  Diagnostic (reports `DL.snoc` report) files
 {-# INLINE addReport #-}
 
 #ifdef USE_AESON
diff --git a/src/Error/Diagnose/Report/Internal.hs b/src/Error/Diagnose/Report/Internal.hs
index 6b487a8..dd0c135 100644
--- a/src/Error/Diagnose/Report/Internal.hs
+++ b/src/Error/Diagnose/Report/Internal.hs
@@ -25,6 +25,8 @@ module Error.Diagnose.Report.Internal where
 import Data.Aeson (ToJSON(..), object, (.=))
 #endif
 import Control.Applicative ((<|>))
+import qualified Data.Array.IArray as Array
+import Data.Array.Unboxed (Array, IArray, Ix, UArray, listArray, (!))
 import Data.Bifunctor (bimap, first, second)
 import Data.Char.WCWidth (wcwidth)
 import Data.Default (def)
@@ -33,9 +35,9 @@ import Data.Function (on)
 import Data.Functor ((<&>))
 import Data.HashMap.Lazy (HashMap)
 import qualified Data.HashMap.Lazy as HashMap
-import qualified Data.HashMap.Lazy as IntMap
 import qualified Data.List as List
 import qualified Data.List.Safe as List
+import Data.Maybe
 import Data.Ord (Down (Down))
 import Data.String (IsString (fromString))
 import qualified Data.Text as Text
@@ -44,6 +46,10 @@ import Error.Diagnose.Style (Annotation (..))
 import Prettyprinter (Doc, Pretty (..), align, annotate, colon, hardline, lbracket, rbracket, space, width, (<+>))
 import Prettyprinter.Internal (Doc (..))
 
+type FileMap = HashMap FilePath (Array Int String)
+
+type WidthTable = UArray Int Int
+
 -- | The type of diagnostic reports with abstract message type.
 data Report msg
   = Report
@@ -153,7 +159,7 @@ err = Report True
 prettyReport ::
   Pretty msg =>
   -- | The content of the file the reports are for
-  HashMap FilePath [String] ->
+  FileMap ->
   -- | Should we print paths in unicode?
   Bool ->
   -- | The number of spaces each TAB character will span
@@ -310,7 +316,7 @@ groupMarkersPerFile markers =
 prettySubReport ::
   Pretty msg =>
   -- | The content of files in the diagnostics
-  HashMap FilePath [String] ->
+  FileMap ->
   -- | Is the output done with Unicode characters?
   Bool ->
   -- | Is the current report an error report?
@@ -367,7 +373,7 @@ splitMarkersPerLine (m@(Position {..}, _) : ms) =
 -- |
 prettyAllLines ::
   Pretty msg =>
-  HashMap FilePath [String] ->
+  FileMap ->
   Bool ->
   Bool ->
   -- | The number of spaces each TAB character will span
@@ -467,34 +473,48 @@ prettyAllLines files withUnicode isError tabSize leftLen inline multiline lineNu
        in prefixWithBar colorOfFirstMultilineMarker <> prefix <> fold (List.intersperse prefix $ showMultilineMarkerMessages multiline)
 
 -- |
-getLine_ :: HashMap FilePath [String] -> [(Position, Marker msg)] -> Int -> Int -> Bool -> (IntMap.HashMap Int Int, Doc Annotation)
-getLine_ files markers line tabSize isError = case List.safeIndex (line - 1) =<< (HashMap.!?) files . file . fst =<< List.safeHead markers of
-  Nothing -> (mempty, annotate NoLineColor "<no line>")
-  Just code ->
-    let (tabs, code') = indexedWithTabsReplaced code
-     in ( tabs,
-          fold $
-            code' <&> \(n, c) ->
-              let colorizingMarkers = flip
-                    filter
-                    markers
-                    \(Position (bl, bc) (el, ec) _, _) ->
-                      if bl == el
-                        then n >= bc && n < ec
-                        else (bl == line && n >= bc) || (el == line && n < ec) || (bl < line && el > line)
-               in maybe id ((\m -> annotate (MarkerStyle $ markerColor isError m)) . snd) (List.safeHead colorizingMarkers) (pretty c)
-        )
+getLine_ ::
+  FileMap ->
+  [(Position, Marker msg)] ->
+  Int ->
+  Int ->
+  Bool ->
+  (WidthTable, Doc Annotation)
+getLine_ files markers line tabSize isError =
+  case safeArrayIndex (line - 1) =<< (HashMap.!?) files . file . fst =<< List.safeHead markers of
+    Nothing ->
+      ( mkWidthTable "",
+        annotate NoLineColor "<no line>"
+      )
+    Just code ->
+      ( mkWidthTable code,
+        flip foldMap (zip [1 ..] code) \(n, c) ->
+          let cdoc =
+                ifTab (pretty (replicate tabSize ' ')) pretty c
+              colorizingMarkers = flip filter markers \case
+                (Position (bl, bc) (el, ec) _, _)
+                  | bl == el ->
+                    n >= bc && n < ec
+                  | otherwise ->
+                    (bl == line && n >= bc)
+                      || (el == line && n < ec)
+                      || (bl < line && el > line)
+           in maybe
+                id
+                ((\m -> annotate (MarkerStyle $ markerColor isError m)) . snd)
+                (List.safeHead colorizingMarkers)
+                cdoc
+      )
   where
-    indexedWithTabsReplaced :: String -> (IntMap.HashMap Int Int, [(Int, Char)])
-    indexedWithTabsReplaced = goIndexed 1
+    ifTab :: a -> (Char -> a) -> Char -> a
+    ifTab a _ '\t' = a
+    ifTab _ f c = f c
 
-    goIndexed :: Int -> String -> (IntMap.HashMap Int Int, [(Int, Char)])
-    goIndexed _ [] = (mempty, [])
-    goIndexed n ('\t' : xs) = bimap (IntMap.insert n tabSize) (replicate tabSize (n, ' ') <>) (goIndexed (n + 1) xs)
-    goIndexed n (x : xs) = bimap (IntMap.insert n (wcwidth x)) ((n, x) :) (goIndexed (n + 1) xs)
+    mkWidthTable :: String -> WidthTable
+    mkWidthTable s = listArray (1, length s) (ifTab tabSize wcwidth <$> s)
 
 -- |
-showAllMarkersInLine :: Pretty msg => Bool -> Bool -> (Doc Annotation -> Doc Annotation) -> Bool -> Bool -> Int -> IntMap.HashMap Int Int -> [(Position, Marker msg)] -> Doc Annotation
+showAllMarkersInLine :: Pretty msg => Bool -> Bool -> (Doc Annotation -> Doc Annotation) -> Bool -> Bool -> Int -> WidthTable -> [(Position, Marker msg)] -> Doc Annotation
 showAllMarkersInLine _ _ _ _ _ _ _ [] = mempty
 showAllMarkersInLine hasMultilines inSpanOfMultiline colorMultilinePrefix withUnicode isError leftLen widths ms =
   let maxMarkerColumn = snd $ end $ fst $ List.last $ List.sortOn (snd . end . fst) ms
@@ -505,19 +525,23 @@ showAllMarkersInLine hasMultilines inSpanOfMultiline colorMultilinePrefix withUn
    in -- get the maximum end column, so that we know when to stop looking for other markers on the same line
       hardline <+> dotPrefix leftLen withUnicode <+> (if List.null ms then mempty else specialPrefix <> showMarkers 1 maxMarkerColumn <> showMessages specialPrefix ms maxMarkerColumn)
   where
+    widthAt i = 0 `fromMaybe` safeArrayIndex i widths
+    widthsBetween start end =
+      sum $ take (end - start) $ drop (start - 1) $ Array.elems widths
+
     showMarkers n lineLen
       | n > lineLen = mempty -- reached the end of the line
       | otherwise =
         let allMarkers = flip filter ms \(Position (_, bc) (_, ec) _, _) -> n >= bc && n < ec
          in -- only consider markers which span onto the current column
             case allMarkers of
-              [] -> fold (replicate (IntMap.lookupDefault 0 n widths) space) <> showMarkers (n + 1) lineLen
+              [] -> fold (replicate (widthAt n) space) <> showMarkers (n + 1) lineLen
               (Position {..}, marker) : _ ->
                 annotate
                   (markerColor isError marker)
                   ( if snd begin == n
-                      then (if withUnicode then "┬" else "^") <> fold (replicate (IntMap.lookupDefault 0 n widths - 1) if withUnicode then "─" else "-")
-                      else fold (replicate (IntMap.lookupDefault 0 n widths) if withUnicode then "─" else "-")
+                      then (if withUnicode then "┬" else "^") <> fold (replicate (widthAt n - 1) if withUnicode then "─" else "-")
+                      else fold (replicate (widthAt n) if withUnicode then "─" else "-")
                   )
                   <> showMarkers (n + 1) lineLen
 
@@ -532,15 +556,15 @@ showAllMarkersInLine hasMultilines inSpanOfMultiline colorMultilinePrefix withUn
             allColumns _ [] = (1, [])
             allColumns n ms@((Position (_, bc) _ _, col) : ms')
               | n == bc = bimap (+ 1) (col :) (allColumns (n + 1) ms')
-              | n < bc = bimap (+ 1) (replicate (IntMap.lookupDefault 0 n widths) space <>) (allColumns (n + 1) ms)
-              | otherwise = bimap (+ 1) (replicate (IntMap.lookupDefault 0 n widths) space <>) (allColumns (n + 1) ms')
+              | n < bc = bimap (+ 1) (replicate (widthAt n) space <>) (allColumns (n + 1) ms)
+              | otherwise = bimap (+ 1) (replicate (widthAt n) space <>) (allColumns (n + 1) ms')
             -- transform the list of remaining markers into a single document line
 
             hasSuccessor = length filteredPipes /= length pipes
 
             lineStart pipes =
               let (n, docs) = allColumns 1 $ List.sortOn (snd . begin . fst) pipes
-                  numberOfSpaces = sum [IntMap.lookupDefault 0 x widths | x <- [n .. bc - 1]] -- bc - n
+                  numberOfSpaces = widthsBetween n bc
                in dotPrefix leftLen withUnicode <+> specialPrefix <> fold docs <> pretty (replicate numberOfSpaces ' ')
             -- the start of the line contains the "dot"-prefix as well as all the pipes for all the still not rendered marker messages
 
@@ -555,7 +579,7 @@ showAllMarkersInLine hasMultilines inSpanOfMultiline colorMultilinePrefix withUn
 
                   lineLen = case lastBeginPosition of
                     Nothing -> 0
-                    Just col -> sum [IntMap.lookupDefault 0 x widths | x <- [bc .. col - 1]]
+                    Just col -> widthsBetween bc col
 
                   currentPipe =
                     if
@@ -639,3 +663,8 @@ prettyAllHints (h : hs) leftLen withUnicode =
 
     noteMessage (Note msg) = msg
     noteMessage (Hint msg) = msg
+
+safeArrayIndex :: (Ix i, IArray a e) => i -> a i e -> Maybe e
+safeArrayIndex i a
+  | Array.inRange (Array.bounds a) i = Just (a ! i)
+  | otherwise = Nothing