bug#44007: 26.3; Strange shell mode performance

[Herman@debbugs.gnu.org, Géza <geza.herman@gmail.com>]

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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).

And it also speeds up, if I don't press enter, but constantly move the 
mouse (using the X version).

It seems that emacs waits for some kind of event here (hence the low cpu 
utilization). But if there's some event (mouse, or an unprocessed 
input), then it speeds up (and uses 100% cpu).

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