It's not that; or not only that. For a while scrolling on my laptop happened at ~2 FPS. Granted, my laptop is not very fast, but plenty of fast enough for compiling stuff and even some 3D games and it was slower than any other website I've ever used by a considerable margin. I can't be bothered to check what the performance is like now, but for a long time the entire thing was slow, janky, and objectively just didn't work well.

They could implement all the ads and user-hostility into Old Reddit just find and it would still be fast. In fact, I bet that Old Reddit with ads and trackers will still be faster than the New one without any of that.