I never had such issues till I downloaded the offline version. Now I’ve cleared Firefox own internal download history (rather than clicking the “clear downloads” box on SC) the problem is resolved.
When I reported earlier it took 10 seconds or so to load a page. Now it is back to less than a second. Firefox for me is not slower than Chrome. If anything it seems the other way around.
On the other issue: I tried the offline use option also in Chrome and also a fresh install of Brave, but had no luck with both either. This function has not worked for me since it first launched years ago, and neither does it for many others, judging by a google search. Perhaps it’s worthwhile adding a note that this functionality may not work, instead of the affirmation that “Your browser supports Progressive Web Apps!” followed by a green tickbox. (Edit: it also doesn’t work on Opera.)