bug#44007: 26.3; Strange shell mode performance

[Eli Zaretskii]

> From: Herman@debbugs.gnu.org, Géza <geza.herman@gmail.com>
> Date: Fri, 16 Oct 2020 12:44:32 +0200
> 
> It reproduces for me, both 26.3 and 28.0.50. And both for the X and 
> console version (by console version, I mean the usual version, but with 
> DISPLAY unset).
> 
> I attached a text file (hopefully it gets through).
> 
> cat-ing this file takes ~18 sec. But with enter, it takes just ~3 sec 
> (these measurements are done with the console version). It behaves like 
> this 100% of the time (it's a more reliable test than the "seq" test).

Does it help to play with the value of process-adaptive-read-buffering?
Or with read-process-output-max?