Has anyone else experienced something similar? Any tips to fix?
In the end, only the Google chrome and gmail teams can answer the true why, everything else is guesswork on everyone's part.
As to youtube, as with @PaulHoule) I too have not noticed any slowness, but my use of youtube in firefox is only to obtain the URL to hand off to yt-dlp (youtube-dl successor). Watching videos with my local client player is worlds superior to playing a stream in any browser (Firefox/Chrome/otherwise) that I simply do not play the streams in Firefox from youtube at all.
The really short version is go to about:config and disable security.tls.enable_0rtt_data; the pretty short version is if you need a new HTTP/2 connection to make a POST request, and the server supports TLS 1.3 Early Data (aka 0rtt), and Firefox negotiates Early Data, it won't send the POST until an inactivity timeout occurs on the TLS socket. In Firefox 94 (in release channel 2020-11-02), that timeout was about 60 seconds; looks like the same for Firefox 95 (in release channel as of 2020-12-07). Firefox 96 (currently available as Nightly, expected release 2021-01-10) had a shorter timeout of 4 seconds, but also has a bug fix as of 2021-11-26.
Of course, it's difficult to tell which client is which during the TLS handshake, so you can't really offer most clients TLS 1.3 Early Data and not to broken versions of Firefox; not that Chrome uses TLS 1.3 Early Data.
I'm not too bothered by sponsored segments in the videos themselves. If they annoy me a couple of right arrow presses are enough to skip them.
This happens even with all my browser extensions turned off.
I find it very frustrating.
Youtube also takes ~30 seconds to fully load, too, versus <3 in Chromium.