I personally find Fastmail's spam handling a lot worse than GMail's: and I get a lot more spam in my GMail address, due to it being a 22 year old address, which I still use for initial sign-ups, before then changing account emails to my fastmail one after a few months.
Very likely a lot of the difference is the types of email each address is getting.
Very likely a lot of the difference is the types of email each address is getting.