While testing most recent commits from main I noticed that noop builds were sustantially slower in the last weeks.

After dissecting for the past few months, I could pin down to commit 9105a43, from #8447, cc @rgrinberg @Alizter as PR author and reviewer.

The regression makes the noop build in Ahrefs case almost 2x slower:

# using commit 9105a43b719cc66aefd7c7f84816bd1a77f236c7
$ time ../dune/_build/default/bin/main.exe build @fe.all
                                             
real    0m43.125s
user    0m41.054s
sys     0m2.092s

# using commit 9c41108359ac93d369a6e78da88ffbde8b1f8ca9
$ time ../dune/_build/default/bin/main.exe build @fe.all
                                             
real    0m22.828s
user    0m21.099s
sys     0m1.760s

All other things in the measurements above are equal. I am building the dune exe using dune build bin/main.exe and calling the exe directly as shown in the time commands above.

I will investigate more tomorrow to see if I can identify which particular rules are slowing down, in case there are any other info I can provide, please lmk.