Perhaps, rather than saying that older operating systems felt faster, and then getting into the weeds debating that (or quoting cold, hard numbers) we should change our terminology: I’d say that older operating systems felt ‘snappier’, which I think is quite different from ‘faster’.
(Side note: I would do unspeakable things for a macOS 9 GUI on a modern, lean OS, plus some cloud integration. That’s all I want, until I think of something else).