I have occasionally run into long pages that load the top of the page normally, but then I get a pinwheel while the rest of the page loads. This however has been alleviated in Safari Technology Preview Release 132 (Safari 15.4, WebKit 17613.1.1.5). I don't know how that corresponds to the Safari 15 that was released for Big Sur as I am running Monterey.

Other than that it is FAST but it may appear that nothing is happening when it is rendering a complex web page then suddenly it appears.

If we knew what it was we were doing, it wouldn't be called research, would it?

— Albert Einstein