On Wed, 10 Nov 2021 16:08:09 +0200, Eli Zaretskii wrote:
> Building Emacs with -O0 should never make Emacs so slow as described
> by the OP.  Disabling optimizations generally makes Emacs about 2.5 to
> 3 times slower, but that's still a far cry from response time of a
> second or two to C-s.

Yes, I have no problem with -O0 on Emacs master till last week
and also on 28, 27, 26.

On Wed, 10 Nov 2021 14:53:00 +0200, Eli Zaretskii wrote:
> Does this happen with any text in any buffer?  If the problem is
> limited to some buffers, please try to figure out which buffers
> exhibit the problem.

It happens on any buffer.

> Also, please show the information collected by report-emacs-bug about
> your build details.

What report-emacs-bug creates is attached below.  This was made
by Emacs from git head, built by `make bootstrap' a moment ago.

Probably what I should do could be bisecting the recent commits.

Thanks.