I've noticed this too.
I've found Ubuntu to be lagging since 7.10 myself and it seems others have noticed
Personally, it's getting on my nerves, and I'm going to be giving Xubuntu 9.10 one
chance to impress me before I dump it for something more svelte, I'm so ticked.
That said, I remember installing Slackware 8 once and although it was quite snappy, window drawing was noticeably slower than the Windows 95(!!) it was sharing a partition with.
Most likely X itself is the culprit no matter what distro you're into, and some folks have pondered what to do about it
Although X is so much part of the whole Linux experience, it's hard seeing it being wholesale replaced any time soon, even with something better.
That doesn't stop me from hoping it happens, though.
It's been a long time coming...