@ekaitz_zarraga

In fact, most of times I build a single program or library in jehanne and it takes few seconds.

The kernel is the slowest component (except of gcc/binutils, obviously), but mostly because to create the initrd, it starts a previously compiled kernel in qemu and copy the required binaries as served by a 9P2000 local fs.
Iow, not something you would optimize with make.

Obviously 10 or even 20 files are not too many but I'm quite surprised it take your machine minutes to compile them.