I was just reading through these posts and thought I'd add a little info (hope it's somewhat relevant). I am currently running a first gen iPhone on 1.1.4 with Summerboard and many apps (GBA, Gameboy, NES, SNES, SSH, Boss Prefs, Video Recorder, iBlackJack, Chuzzle, Chess, Squid, MIM and a few others). I am using the Glass Orb theme. Right now the UI on this phone runs GREAT. VERY quick, smooth and snappy. I also have a 3G with 2.1 also Jailbroken. Pretty much the same apps and the same theme (except running Winterboard). The 3G is choppy, slow to open apps (especially Settings, Photos and SMS).
Now, I don't know if this is due to Winterboard or 2.1 just sucks. But it could be a combination. I have pretty much decided that I'm going to sell my 3G and keep 1.1.4 for now as it is nearly a perfect phone but for the lack of 3G.
Is anyone else experiencing issues with 1.1.4? As far as I could tell the discussion involves 2.x. I'd be interested to hear what commonalities may be causing slowdowns.