I run KDE 4.4.4 on Gentoo Linux. My system has a Nvidia GeForce Go 7900 GS GPU with the latest 256.29 beta drivers from Nvidia. I noticed that minimizing windows has a nice smooth animation, but when expanding them, the animation seems to stutter. Today, I just tried repeatedly minimizing and expanding the System Monitor, which gave me a clue as to why this is happening. My Intel Core Duo T2400 CPU goes to 50% loading during this exercise on both cores according to System Monitor. Opening another window, such as Chromium and doing this exercise brings both cores to 60% load. Doing Alt+Tab switches between the windows repeatedly brings both cores to 40% load.

This effect is still present even when compositing/desktop effects are disabled. Switching from Kwin to Openbox under KDE and repeatedly minimizing and expanding System Monitor will also exhibits this issue, but the effect has been more than halved, where instead of exhibiting 50% load on both cores, there is only about 20% load on both cores. Idle loads are roughly 3%. I also tried doing this under LXDE and there is a noticeable increase in load when repeatedly minimizing/expanding System Monitor, but the increase is barely anything at all. Repeatedly minimizing and expanding Task Manager on my desktop that runs Windows 7 Professional does not seem to affect CPU load at all.

Does anyone know why this is happening?

I believe this is due to the way the X protocol works, where applications that are not on screen are not painted.

It is done in this manner because of historical issues I believe, where terminals simply displayed applications running on a server, and reducing network utilisation by not sending minimized applications was needed.

Windows simply places applications far off the screen when minimizing them, thus restoring them to the screen is much less CPU intensive as they simply have to be moved, rather than the whole window redraw as with X.

Would filing a feature request with the creators of xorg-server requesting for some kind of mechanism that desktop environments could use to have these things still be painted be a good idea then?

I do not think in this case ( after performing further tests locally with Opera, and using top in Konsole to monitor CPU usage ) that the X server is to blame.

At least in my case, with Intel graphics and compositing enabled, I didn't see a significant rise in CPU usage. ( only by about 15% with a single core system ) This could be related to resizing of window contents during minimization not being optimised by the graphics drivers however.

Interesting.

I've noticed that doing things in KDE that typically do not cause high cpu usage in other desktop environments causes high cpu usage in KDE. E.g. just a mousing over the task manager (without compositing even) causes moderate cpu usage (same for smooth tasks and similar plasmoids), and likewise for alt-tabbing without compositing. When compositing is enabled Xorg and Kwin are generally the culprits, but when compositing is disabled the culprit is typically plasma-desktop instead.