I run Ubuntu on my work laptop (with i3 WM). Despite my desktop having a very measly dual-core Intel Pentium G3258 (a 10 year old budget CPU, although admittedly overclocked to the max) and my laptop having a much more recent quad-core i7, the Gentoo box absolutely flies compared to the Ubuntu one.
I've also successfully run Gentoo on an old Atom powered netbook. The only problem is it's too slow to run Firefox and the "modern web", but that can't be helped. I compiled the whole system on the netbook, although nowadays I would set up cross-compilation if I were to do it again.
Upgrades where you are expected to do something rarely happen and even then it is fairly simple.
The major downside is when you want to try out something - it takes time usually, so it is not instant as with binary distros and other OSes. For now, when I am not willing to wait, I just use Fedora VM to test stuff first.
Totally fine. You should have access to all the major packages, WMs, DEs, etc. Once you've installed everything, it will be like any other distro.