I too have noticed some lag in the 5.0.1 untether.
But I pretty much figured out how to make my phone really smooth. The only thing I cannot seem to fix is a very slight delay in loading applications....and a very small amount of lag when closing apps with the home button to a full SpringBoard page. But that is it.
I use Speed Intensifier addon in WinterBoard (without the Speed Intensifier tweak installed..just drains the battery). Within WinterBoard I use the SI UnderClock addon and I really think it has really improved the animations in my phone a decent bit.
I am overall satisfied with the smoothness of my phone now that I have figured out how to keep it going well.
I have over 60 Mobile Substrate addons installed and well over 100 extensions installed!
Yes I know...that is a sh** load of tweaks rite?!
Well....the best way to keep your device running smooth is to install all your tweaks at one time.
Seriously.
I extract all my Cydia packages to a folder on my desktop.
Then then after restore and jailbreak I drag an drop the entire contents of the folder to the Autoinstall folder..reboot twice.
BiteSMS is one of the smoothed applications I have on my SpringBoard...that tweak is literally near perfection in my book...but I do always use the latest beta from the biteSMS repo instead of the public version..which I think is 6.2.
My phone is like ridiculously fast and smooth when you really consider how much I actually have installed. It's a lot of tweaks...
Obviously the more Mobile Substrate addons u have installed the slower your device will get.
When I install *almost* all of my packages at one time (doesn't really matter how you do it) my phone can function very well with an a** load of tweaks installed.
And when I use the SI UnderClock addon within WinterBoard...it's even smoother.
Love it.