According to "top", /usr/bin/X is using between 30% and 60% of my CPU (and can go higher). this is on an AMD A6-3670 (quad core) system running Kubuntu (13.04 beta), and my new nVidia G210 graphics card with proprietary drivers. I had the same problems with the onboard GPU in 13.04 and 12.10 before it.
Eg: PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 1357 root 20 0 321m 207m 49m R 48.9 1.3 214:58.86 Xorg
Why would X CPU be so high, and what can I do about it?