I'm sorry, but I disagree. I don't hate TW or Sense, but they aren't smooth at all. My Fascinate has quite a bit of lag even when just swiping through homescreens or opening/closing apps. I rooted it and am now using ADW Launcher (came baked into the ROM I flashed) and all of that lag is gone. Yes I am using a custom kernel, but it is not OC'ed and it isn't Voodoo-enabled. So not much changed, other than losing TW, yet my performance increased.
TW and Sense add features, but they aren't very smooth.
A) I didn't say Sense was smoother - I said it was considered better. For a long time, it was.
B) You are running a custom ROM and comparing them to a custom launcher, not the default Google launcher. Many have commented that they wonder why Google can't make the default launcher as smooth as ADW/Launcher Pro. Not the same comparison as, say, the Nexus One to the Fascinate.
I had a Droid, which ran pure, stock Android. Granted, its a year old, and doesn't have the fastest CPU, but even OC'd it was choppy as all hell. When I briefly had the Epic, it was considerably better - better than Android running on my HD2 (which admittedly is not a fair comparison either). Smoothness is just not stock Android's forte, reviews all will tell you this.