Re: M-x grep is too slow

[Richard Stallman]

I am not surprised that font-lock is responsible, but we need to
find out what it's doing that takes so long.

If you follow my suggestion to run under GDB, type C-z, and then type
xbacktrace, you'll find out which functions of font-lock mode
are running most of the time.  If you put a call to (debug) in
one of them, you can step through it and see whether it takes
a long time, and what it is doing.

It would be useful to check whether the font lock keywords
used for compile mode have changed in the past few months.
If so, that change might be at fault.