Strange shell mode performance

["Herman, Géza" <geza.herman@gmail.com>]

Hi all,

I'm experiencing strange shell (comint) mode performance. If I cat a 10 
MB file, it takes 1:20 to cat it. Emacs uses only 30% cpu. But, if I 
press enter after I send the cat command, it speeds up, and it only 
takes 8 sec (emacs cpu usage goes up to 100%).

Same thing: if I execute "seq 100000" in a small emacs window, it takes 
~3 seconds. But, if I press enter after executing seq, it takes only 0.4 
sec (this is not 100% reproducible, sometimes pressing enter doesn't 
make a difference).
(I executed comint-clear-buffer before measurements)

It happens with 26.3 and ~3-week-old master branch as well, without any 
extra config (emacs -Q).

Any ideas what causes this? Would it be possible to always have the 
better performance without needing to press enter?

Thanks,
Geza