Gmail is subject to specific targeting by spammers in a way that fastmail is not. The returns for spending weeks or months finding a niche way through Gmail's filters are justified by the number of gmail addresses that can be targeted, which is probably 3 orders of magnitude larger than the total number of fastmail subscribers.

Are they the same target email address though? If not, surely that's not a fair test if the gmail account has been around for 10+ years or so?

(I have the same situation, an 18-year old gmail account and a 6 year old fastmail account, but the reason I don't get ANY spam at all in the fastmail account is I only use it for certain things and it's much newer, so I'd argue at least in my case, that's not a fair comparison).

I was on fastmail for about a year. It didn't filter nearly as much, and Fastmail as a service was constantly experiencing outages. Pretty much weekly. I would say it's an almost unanimously inferior service.