That's a fair point. I've always found Firefox pretty speedy, but I can't judge other people's experiences and I don't think it's possible to objectively measure subjective user experience. Every piece of software has people who have unusually bad experiences and I don't know how to judge which are reliable and representative. The best you could do is polls, which are not reliable.

I don't know how to tell the difference between "Firefox is slow" and "Firefox has an undeserved reputation for being slow."