OK, this is rather difficult for me to explain, but I will try to explan this issue as detailed as possible.
Because I am a developer I frequently disable the theme service, or just run in Aero basic mode on Windows 7 to make sure all my CPU and GPU time is not being wasted on aero and fancy stuff.
Although I am aware of the benefits of DWM, and GPU acceleration, GDI+ is NO LONGER hardware accelerated in Windows 7 due to the new Direct2D (which GDI+ does not use at all, only upcoming applications will). This means that all onscreen surfaces (even in classic mode) paint INCREDIBLY slower than before, especially slower than in XP and Vista, since they both supported the same hardware acceleration of GDI+. (No, on the contary to popular speculation, GDI+ IS indeed hardware accelarated).
This means that we are being forced to write applications for either WPF or Direct2D, meaning the old Windows Forms apps are becoming incredibly slower. It doesnt matter what mode you use (aero, classic,) it will still perform incredibly slower than in xp and vista because of GDI+ acceleration has been removed.
This has happend to four up to date, new spec pcs, and they all perform exactly the same way.
A good way of testing this is by opening .net form apps and making their opacity to about 95%. Comparing window moves on xp, vista, and 7, 7 chugs HEAVILY and cuts off many regions of the window. XP and Vista perform in nearly the exact speed.
I am sure that many others will be able to reproduce this issue if they know what they are looking for, and the results are shocking.
A simple solution, Enable acceleration of traditional GDI model like in Vista and XP. It was fine then, dont break it so we have to move to Direct2D or WPF.