The thing I noticed in the slow new Twitter is that typing a new status seems to involve continuous autofill-like behavior for each character typed, which seemed to introduce round-trip lag (or javascript-parsing, or...) and causes characters to appear at a rate of ~1/second. Unusable.
And this is one way that dominant sites fall: business considerations are given priority over the user experience. Maybe they just have Flickr-itis and just can't adapt to leaving well enough alone, but for the time being I'm on old-Twitter and not contributing much to the Twitterverse until this is sorted out. If it never winds up being fixed, I've got a head start in living without them.
There is also a possible Second System Effect amongst Twitter's UX gods. It's interesting that after having switched to the new Twitter a couplefew weeks ago, last week I started seeing a "Wanna switch back to the old Twitter?" header. Of course I did, but to me this possibly points to a level of complaints that I did not realize.