[incrudible]

Classic mode does not use compositing at all, but the drawing routines of GDI etc were hardware accelerated already. Keeping a buffer around for every window for compositing could become a big performance problem and was part of the reason why Vista was considered slow. Mac OS X had this problem too.

[rootw0rm]

I was curious myself, this link is a bit of an introduction to the mess:

https://learn.microsoft.com/en-us/windows/win32/direct2d/com...

"GDI is hardware accelerated on Windows XP, and accelerated on Windows 7 when the Desktop Window Manager is running and a WDDM 1.1 driver is in use. Direct2D is hardware accelerated on almost any WDDM driver and whether or not DWM is in use. On Vista, GDI will always render on the CPU."

[kasabali]

Here's a more honest and direct introduction than Microsoft itself could wrote: https://forums.cockos.com/showthread.php?t=55358

Especially follow the YouTube links demonstrating the problem: https://www.youtube.com/watch?v=ay-gqx18UTM https://www.youtube.com/watch?v=ToFgYylqP_U