Thanks for looking into it.

I have tried to reproduce the slowdown on Ubuntu with Emacs 28 and 27.2 and
compared against my current patched 27.1 Emacs. The conclusion is that
something improved in a recent version of Emacs and the patch is not moving
the needle on this.

There is still a super-liner cost somewhere. But as a user I only hit this
when accidentally displaying a large file, and current behavior is
interactive enough that I can kill the process running in the shell.

Once I get a chance I would also like to try on Mac OS X where the problem
was more pronounced.

Thanks,

Anton

|--------------------+---------------------------------------+----------------|
| GNU Emacs 27.2     | time python3 -c 'print("x"*1024*256)' | real
0m20.805s |
| GNU Emacs 28.0.50  | time python3 -c 'print("x"*1024*32)'  | real
0m0.315s  |
| GNU Emacs 28.0.50  | time python3 -c 'print("x"*1024*64)'  | real
0m1.151s  |
| GNU Emacs 28.0.50  | time python3 -c 'print("x"*1024*128)' | real
0m6.168s  |
| GNU Emacs 28.0.50  | time python3 -c 'print("x"*1024*256)' | real
0m22.153s |
| GNU Emacs 27.1*    | time python3 -c 'print("x"*1024*32)'  | real
0m0.237s  |
| GNU Emacs 27.1*    | time python3 -c 'print("x"*1024*64)'  | real
0m1.108s  |
| GNU Emacs 27.1*    | time python3 -c 'print("x"*1024*128)' | real
0m5.172s  |
| GNU Emacs 27.1*    | time python3 -c 'print("x"*1024*256)' | real
0m24.369s |
| GNU Emacs 28.0.50* | time python3 -c 'print("x"*1024*32)'  | real
0m0.234s  |
| GNU Emacs 28.0.50* | time python3 -c 'print("x"*1024*64)'  | real
0m0.978s  |
| GNU Emacs 28.0.50* | time python3 -c 'print("x"*1024*128)' | real
0m4.716s  |
| GNU Emacs 28.0.50* | time python3 -c 'print("x"*1024*256)' | real
0m19.047s |