config: adjust default mux_output_parser_buffer_size

This size seems to be ~sweet spot for `time cat bigfile`, cutting the
time in half from the prior value.  Making the buffer smaller doesn't
improve things, nor does making it bigger; this is balancing the latency
of applying the update to the model with the back-pressured parsing
latency.

config/src/lib.rs

@@ -1790,7 +1790,7 @@ impl Config {
 }
 
 fn default_mux_output_parser_buffer_size() -> usize {
-    10 * 1024 * 1024
+    128 * 1024
 }
 
 fn default_ratelimit_line_prefetches_per_second() -> u32 {