I used to suffer it all the time.
Back when I had an Athlon XP 2.4Ghz (Thus Single Core) with 512Mb DDR1, GeForce FX5600 256Mb, I suffered this problem whether it's the Login Screen, or in-game, it was always maxing at 98-100%. What makes it worse is expanding the game screen to have more Black Desktop space - maximising the window completely just grinded it pretty much to a halt. Also, removing excess windows, or items on the desktop, generally helped speed things along (Discovered this when I got bored one day, and collected 50 Player Profiles and minimised them into a collection on my screen, veeeery slow).
Now on my Athlon 64 X2 5Ghz (Dual Core) with 2Gb DDR2, and a GeForce 7950GT, it's... in fact agreable, it's at least halfed. 50-60% Overall usage when maximised, when sized to 1280x720 it's 30-40% at most.
Definitely worth upgrading if you're on an ancient Single Core.
I just realised I'm still using the "Reduce Display Framerate" while I was testing screen recording once, my character glides with it enabled... but disabled, it's noticably choppy (Leaving it enabled now!), and CPU usage rockets back up to 60%.
Last but not least, turning on and off sounds used to have a notable impact on performance on my old Single Core, but it doesn't appear to be a problem with this newer machine.
Just a few observations put out there, in case it helps anyone... not that sacrificing sound is particularly acceptable, heh.
Edit: Ah damnit, and one last thing, if you're an MSN Messenger Client user, that lags my Client quite noticably as well! Even minimising doesn't help, I have to cross it off and send it to the System Tray... I don't believe the Conversation Windows have an impact... probably the crappy Flash Adverts in the Main Window is what buggers things up. And yes, I've heard of Trillian and Pigeon
