This one also loaded basically instantly for me as well, but for example this one: https://mameson.com/experiment/glsl/clouded_cracker_2/cloude... took ~15 seconds to load and then runs at 9 fps on a pretty powerful dev machine. (granted, it is running a bit slow and I'm also running a vm at the same time, so maybe that's the cause)
Ok, it turns out it's all my fault: fps was terrible due to the vm I was running simultaneously, it oes to 60 fps with vm turned off; and the loading time was because I was downloading it through my company's vpn.