I've observed it on both laptops I own!
HP Compaq nx6125 and Ubuntu 11.04 x64 - somewhat solved
Lenovo Thinkpad T500 and Ubuntu 11.10 x64 - help needed!
I'm still struggling with the Thinkpad to get performance level similar to that of 10 y.o. laptops...

All in all a really serious issue with multiple versions of Ubuntu that renders computers with perfectly compatible hardware unusable, as far as out of the box experience is concerned. Troubleshooting resultant issues seems to be a hard case even for users with some experience with installing graphics drivers.

EDIT: I can't really post additional details. Two different ubuntu versions, two laptops, two different set of graph. drivers (OS vs ATI prop.) - all with the same symptoms. Also I can't stress enough how massive the performance degradation is compared to a healthy system. For that reason I ask for input from people who may know roughly what are we dealing with here.

I can post more details if we were to focus on my current Thinkpad T500. In that case my

current system details:

Lenovo Thinkpad T500
Ubuntu 11.10 x64
ATI Mobility Radeon HD 3650 (also see the "What I have already tried" section about Intel graphics tested)
ATI Catalyst 11.10 drivers
OCZ Agility 3 SSD

but!

• same with the default driver for ATI the card
• same with the prop. driver for the ATI card from Jockey (Additional drivers applet)

What I have already tried:

0. Switching to Intel integrated card (Intel GMA 4500M HD) with the default driver - same effects => may indicate not driver related problem but a problem with something of global influence like e.g. nomodeset or other I don't even know about.

1. (What you can read above)
   
2. ATI Catalyst 11.10 and radeon.modeset=0 boot parameter + disabled Wait for VBlank.
   
3. Unity 2D
4. Ubuntu 10.04 LTS tested (ubuntu-10.04.3-desktop-i386.iso): Both live USB and installed version blazing fast! (on the default drivers - without even installing the proprietary fglrx drivers).

re2 a) seems to give me the only significant results (still poor) - perfect Unity elements performance with the same crawling stuttering/lagging when dragging windows around.

re2 b) this happens often http://i17.photobucket.com/albums/b68/Bucic/ubuntuforumsorg/Screenshotat2011-10-28083140.png

re2 c) Sometimes I am able to witness a normal performance when dragging a window around but only for a second or two. When I try to shake it longer it starts to lag and it will keep lagging like that with an increased probability of what you see in the sshot in point re2 b).

re2 d) I can't establish the radeon.modeset=0 influence though. Once it seems to work be smooth with it, the other time - without it. Really can't tell.