The reason it is done client side is to offload the labor of this user-pref to the client.
The server aggressively caches the pages that are served and what you propose would require that each request be generated just for little ole you and would bring anything less than big-blue to it's knees.
Shouldn't happen and aint gonna happen. not today, not tomorrow, not ever.;-)
And as far as a client-side remedy for your UX blues, I don't see much more wiggle room. The page has to render before the filter can be applied and there doesn't seem to be any animation involved.
FWIW - my experience is that the collapse is noticeable (i have an inordinate number of ignored tags) but instantaneous.
Are you browsing on a mobile platform or a computer powered by hampsters on a wheel? ;-)