Building modules take forever


I'm building modules with Rust apps (because this is only way to get them
in f28/f29/f30) and I noticed that building them takes ages (more
specifically, more than 12 hours(!)). I can build all those packages on my
laptop under 1 hour. Even composes take less time.

It seems that MBS has limit of 20 builds in parallel. Is there any reason
for it? Why can't we just build all packages in parallel?