4GB is barely enough. But you can limit the compilation threads to save memory. For big compilations you should reserve around 1.5GB per compilation thread.
As for celeron... You'd better use binary host, at least for big packages (or have your own binary host).
Here I have one old Chromebox for which I flashed coreboot into.
The three first lines show compilation times of nodejs with quite normal compilation settings. On the last line I enabled some ridiculous optimizations, like -funroll-loops and -fipa-pta but also -lto (which probably contributes the most of the compilation time increase). I've retired this box now, but I might give it a new life as some home automation box.
... and because I had some 8GB DDR3 SODIMM RAM sticks I stuffed the maximum amount in there. If I was on 4GB, I'd use binhost or tune the compilation settings so that the process would use as little memory as possible.
I mean, it can be faster than your average distro on some scenarios. Mostly if you know your way in kernel config.
Though most of its real advantages are in the form of a lean system completely tailored to your needs.
It seems to me most of that Gentoo FUD comes from people that never even tried to install it or gave up because apparently reading a wiki is too hard for them.
Yeah, tools often have compiler-flags. For example, support for other software you have or don't want to have. It's the more simple alternative to autodetection during compile, like mpv does. For example, X11 vs. Wayland.
Thanks, surpassingly, it didn't take long around 6-8 hours. I started at around 12-13 and ended at 19. I did kind of speed run installation and did as fast as possible. I first tried installing gnome or kde, but compiling was too long, so I decided xfce.