For anything lower-spec (like, <4Gb of RAM), Ubuntu absolutely CHUGS because of Snaps. Flatpak has no such issue.
Ironically, Lubuntu (a lightweight Ubuntu fork) worked the best for me while I was using it. No slowness, but I installed pretty much everything using Apt (didn't know about Flatpak back then).
I ended up having it lock up and freeze on the sign-in page though, so I moved on to the slightly heavier Linux Mint.