To be perfectly honest, you could just be less sensitive to it, or you're running on a machine that can brute force through it. Until very recently, Firefox was visibly under-performing compared to Chrome, especially for WebGL related tasks or when running on low powered laptops and mobile devices like my Surface Pro 7.
It's been improving a lot recently though, so much so that I've switched back from Brave to Firefox, but to say that benchmarks are just synthetic tests is being dismissive about Firefox's performance issues. Benchmarks are the only way to quantifiably and scientifically measure performance improvements and regressions. Simply using a browser and declaring whether its fast or slow is inaccurate at best and misleading at worse, and what's okay performance wise for you may not be okay for others.
It also depends highly on your browsing habits. If you're just reading news articles and watching videos, sure you probably won't notice the improvements that these benchmarks show. Nowadays, however, more and more sites are becoming full on applications that are doing way more Javascript operations in the background than a simple document and that's where these benchmark improvements really have an affect on everyday browsing.