Widgets etc. can be disabled as far as I know. Performance issues I have seen thus far were mostly on the server side. On my phone it is fairly smooth now using the PWA in the browser (Firefox Mobile) and that is on fairly modest hardware (4GB of RAM, Unisoc T606 SoC). On my Odroid N2Plus (only slightly beefier SoC than the phone and as much RAM) it performs fairly okay as well (with regular desktop browser). I will not speak about Firefish on my desktop because I am not being fair then.