Hi,

When investigating `re-search-forward' performance in
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=58558 (bug#58558), I
noticed that buf_bytepos_to_charpos is taking most of the CPU time,
according to perf stats.

This was partially caused by `parse-sexp-lookup-properties', but even
after working around the text property issue, buf_bytepos_to_charpos
still shows up on top of the perf profile.

Since one of the apparent bottlenecks in buf_bytepos_to_charpos is

for (tail = BUF_MARKERS (b); tail; tail = tail->next)

which obviously scales with the number of markers in buffer, I decided
to add a cut-off parameter, as in the attached patch (number 50 has no
particular motivation underneath).

Surprisingly, this simple change reduced my Org agenda generation times
from 20 seconds down to 3-4 seconds!

I am sure that my dumb approach is not the best way to improve the
performance, but this place in buf_bytepos_to_charpos is clearly
something that can be optimized.