That used to be true back when the CPU did all of the GUI rendering. But now most all of it is offloaded to the GPU. Any GPU that can render Quake 3 Arena at 120 FPS (and that's ALL of them even Intel IGPs) can wiggle a window around very easily.
Not sure about file opens. Simple applications like GVIM don't seem to have seconds of delay for me, but I know what you mean with things like spreadsheet or word processor files. I guess it is all of the unzipping and XML processing.
Basically, the GPU card vendors could hook any part of the win32 GDI pre windows vista, and they did. Post vista, only a tiny portion of the GDI is accelerated. In theory you can avoid this by writing your application using a more modern API, but the vast majority of native windows applications continue to basically be GDI based due to age, or various GUI toolkits still being GDI based. Worse there are a number of toolkits (or browsers) which implement their own drawing routines rather than calling the system supplied ones.
The final composited results with aero are of course accelerated, but that only really tends to add additional latency. Switching to one of the basic themes makes win7 noticeably more responsive, but also tends to tear a lot. I've got an incredibly high end desktop machine, carefully tweaked/optimized and I can frequently see it the ~1/10 of a second lags while windows update after being maximized/etc. Compared to the 10 year old pretty high end XP machine (with upgraded SSD/etc, and also carefully tuned) it doesn't seem to be faster on basic desktop type operations. Fire up a recent game, or doing builds its massively faster but running word/firefox/whatever the old machine "feels" faster.
(tuned, as in I have a dozen or so, tweaks I've been collecting/researching for the past decade+, on ways to make the machine feel more responsive, it all started with MenuShow delay in win95, and has grown from there and now includes all the usual stuff plus tweaking power profiles, and a bunch of less obvious "feel" things like high DPI mice with fast base speeds).
Not sure about file opens. Simple applications like GVIM don't seem to have seconds of delay for me, but I know what you mean with things like spreadsheet or word processor files. I guess it is all of the unzipping and XML processing.