@dankcushions well, they are OCing the pi4 to 2.16Ghz (cooling tower ? maybe water cooling ?), downclocking the emulated cpu to 30% (so any game that used more than 30% of gamecube's cpu speed is out of reach), and that's still far from perfect. The situation will probably improve when we get affordable SBCs at the level of a snapdragon 845, but we are still probably 2-3 years away from that.

I’ve run retropie on a 3Ghz Pentium 4 with 4G of RAM and it all works. You don’t need too much to get it all going.

With modern equipment, I have an AMD A6-9500 with 4G RAM and built in graphics and that is more than enough

I’ve used lubuntu 16.04LTS on really old builds.
There is a lubuntu 18.04 in 32-bit if you want it. Note that 18.04 needs to be set to using Wayland at time of writing because Xorg gives libretro a hard time.