> i wouldn't expect css gaussian blur of all things to cause slowdowns in some browsers

Gaussian blurs are extremely performance intensive. They need to be calculated for every individual pixel, and the per-pixel computation involves averaging the colors of dozens or hundreds of adjacent pixels. This site applies it to the entire <main>, which spans the entire screen's width and height on mobile, which means that it has several million pixels. That's fine if the browser only has to calculate it once, but when you're scrolling the browser has to perform those calculations for every single pixel on every single frame.

Apparently Chromium and Safari downscale the sampled texture which makes it less accurate but more performant, but Firefox doesn't do that because it prioritizes accuracy over performance, which explains why it only stutters on Firefox.

Last time i checked it's not laggy because they messed with the scroll, but because of some fancy CSS that firefox has trouble with.