On Sat, Jul 06 2024, Eli Zaretskii wrote:

>> What else could cause it in ert-test-run-tests-batch-expensive?
>
> I don't know, and I don't see why we should guess.  Run that under a
> debugger with a breakpoint in balance_an_interval that shows the
> backtrace, and you will see the answer very quickly.

Here are the flame graphs; recorded with

  perf record -g -o ../src/emacs -Q --batch -f igc--collect \
  -l lisp/emacs-lisp/ert-tests.el --eval \
  '(ert-run-tests-batch-and-exit
  "ert-test-\\(parse\\|plist\\|record\\|run-tests-batch-exp\\)")'

Both the MPS and non-MPS version compiled with -g -O2
-fno-omit-frame-pointer.  I don't see the anything very quickly.