When the "Tearing Prevention (VSync)" setting is set to "Automatic" when using the nvidia binary driver, I get screen tearing (on a GTX 760).
If I set it to "Full scene repaints" or "Re-use screen contents" the tearing goes away.
I would expect the "Automatic" setting to do the right thing, but in this case it does not. This seems to be a common source of frustration for many users who haven't figured out the correct setting.
ensure you're not running into bug #322060
the auto-selection on the nvidia blob is "re-use screen content"
Ah sorry, I didn't see that bug report because I didn't notice any CPU load issues but setting KWIN_TRIPLE_BUFFER=1 does fix the tearing issue with the Automatic setting.
*** This bug has been marked as a duplicate of bug 322060 ***
If you've triple buffering enabled in the driver there'll be no additional CPU load.
See the last dupe comments and if you can provide the measured time on failure (notice that you've to unset KWIN_TRIPLE_BUFFER or the measure won't take place)