I just upgraded from Plasma 5.10.0 to 5.10.1 (Arch Linux), Plasma started running with very high CPU. I get about:

* 80% for plasmashell
* 50% for kwin_x11
* 40% for Xorg -nolisten tcp -auth /var/run/sddm/{<random_string>} -background none -noreset -displayfd 18 vt1

After about an hour of troubleshooting, I tracked it down to my background picture, which was set as a slideshow, updating every 10 minutes. After I changed it to a static image, CPU usage dropped to normal. A bug has been filed here.