I thought so. Various versions of NVIDIA drivers and KWin have not worked well together, and the common effect is high CPU usage. I'm not sure whether the bug lies in the NVIDIA driver or in KWin, but my guess is the former. If you google e.g. "KWin NVIDIA CPU" you'll find plenty of posts about this problem. You'll even find posts about it in the SL Forums, if you search.
You may find that if you disable compositing in KWin then the heavy CPU usage will not occur. To try that, you can use the KWin default shortcut Alt-Shift-F12 if it works, otherwise see my blog post Toggle KWin compositing on and off easily from your Desktop
for other ways to do it.