I agree totally, in many places GIMP's UI is both counterintuitive and clumsy but that's not the only reason in that there are significant performance issues. For one, unlike Photoshop, GIMP 'dribbles' its screen refresh rate which is most distracting and annoying.

If the developers found they couldn't easily increase the screen refresh speed after some adjustment then why didn't they delay its updating until the computation had finished (the 'dribbling' being more objectionable than a delay)?

I don't have the time or inclination to become familiar with the huge job of understanding GIMP's source so I've often wondered why its screen refreshing is significantly worse than PS. From observation, PS cleverly only updates the relevant (viewable) changes rather than the full 'effective' screen at any given point in time so changes appear almost instantly. This problem and the UI one put GIMP out of the running as far as I'm concerned.

BTW, re your last paragraph, note my comment about ergonomics in my comment to southerntofu.