> P.S. The above benchmark should be repeated with native-compilation,
> IMO.

I repeated it for "make bootstrap" because I don't know how to only
rebuild the lisp directory with native-compilation.

So basically I did:

for percentage in 0.1 0.75 1.5 ; do
    /usr/bin/time -o make-$percentage.time make -l3.2 -j4 \
 BYTE_COMPILE_EXTRA_FLAGS="--eval '(setq gc-cons-percentage $percentage)'" \
 FAST=true -l3.2 -j4 bootstrap
done

The results are:

* for gc-cons-percentage = 0.1:
2331.71user 86.23system 14:38.08elapsed 275%CPU (0avgtext+0avgdata 641880maxresident)k
4432inputs+742056outputs (18major+14952231minor)pagefaults 0swaps

* for gc-cons-percentage = 0.75:
1223.32user 85.96system 8:06.30elapsed 269%CPU (0avgtext+0avgdata 706240maxresident)k
0inputs+742064outputs (2major+15873839minor)pagefaults 0swaps

* for gc-cons-percentage = 1.5:
961.91user 94.57system 6:32.58elapsed 269%CPU (0avgtext+0avgdata 766348maxresident)k
16inputs+742064outputs (0major+18617902minor)pagefaults 0swaps

[BTW, boostrapping the igc branch requires:
1039.27user 181.75system 7:19.31elapsed 277%CPU (0avgtext+0avgdata 929092maxresident)k
38960inputs+682528outputs (8major+27973694minor)pagefaults 0swaps ]

For fun, I also recorded the times for individual compilation processes.
Here a scatter plot of the result: