I haven't noticed any slowdown at all, and it wouldn't depend on the document size anyways. In fact, I remember Docs slowing down on 30-page files a decade ago, whereas now it handles 100 pages just as fast as 1.

Only the visible portion of the document is rendered (previously in HTML, now in Canvas). Everything before/after is just the document data.

Maybe you started working on longer docs coincidentally around the same time Docs switched to canvas?