Another danger here is testing with the website being the only thing on the PC: some people use their computers like that but many others will have many differe

Another danger here is testing with the website being the only thing on the PC: some people use their computers like that but many others will have many different things open at once: just because it can run at an acceptable FPS on the detected hardware doesn't mean the resource use is acceptable on a system which is multitasking. I usually have >100 tabs in my browser and at least 20 of which are loaded: they cannot all demand what your site demands, even though the hardware is quite high-end.