Unfortunately, here also Firefox + uBO (non-lite) and I get the delay. Or rather, I got the delay until I applied the latest update to the Quick fixes filter. That seemingly took the delay down a little but not completely (don't know if it was for that specific issue) I then applied a rule to My Fitlers that @nicolaasjan provided here. It worked for me--now any YouTube page will load almost instantly for me, like in the olden times. Does not affect me in Brave for some odd reason--Brave has never given me any YouTube trouble to date. So I don't know what the deal is with Firefox + YouTube. But for now: it's OK again.