I have been using the official NVIDIA drivers and the Composite extension in Slackware -current for about a year now. I have been using XFCE, which has a built-in (but rather limited) composite manager, and that has worked good for transparency on my windows, and some shadows.

But using the new XFCE in Slackware --current, the built-in composite manager started giving me problems. I decided to take this opportunity, and move on to using xcompmgr, which has some more features.

Well, everything was working well, until I noticed some lag when I had many windows open. Taking a look at top, I saw X was using 70%+ CPU! Trying some things, I found that when I disabled the shadows, the CPU usage dropped to normal levels. Transparency had no effect on the CPU, however.

So it seems to me that transparency is using the hardware acceleration properly, but shadows are using software rendering.

Is there a way to get shadows to be done in hardware, or have I just missed something along the lines?

I read this today. It might relate to your question. I don't know. Read the part about the 8 Spot test. If nothing else it shows that there are weird issues with graphic rendering that are not widely known.

http://www.lostcircuits.com/video/quadro_fx4500/7.shtml

Well, thanks for the input, but I am not sure how that applies to me, actually.

Nobody here is running Composite?