I had major lag issues from 100% cpu usage. It went back down to normal (Still has spikes of high cpu for a few minutes at a time tho) by disabling use new player models, and disabling the glow for quests/targets, etc.
Also limiting your FPS to 13 frames for both foreground and background free's up some more cpu usage.
You can find limiting your fps in the "System", and "Advanced" selection. On the right side, checkbox both foreground/background boxes and change the sliders near where it says 8.
You can find the "Show new character models" selection in "System", and "Advanced" selection. Uncheck that box.
You can find turning off the glow in "Interface", and "Display" selection. On the very bottom the "Outline Mode!" change that to disabled.