In the graphic options thing, just click "Minimum" and make sure you're on Safe Mode (most bots don't work without safe mode anyway). However, Z-buffering hasn't upped my CPU at all, as far as I can tell, so maybe something else is?
I've had to re-install the bot client and re-upload all my auths before (not just copy+pasting the config, this didn't solve the problem). For some reason, it was running really choppy, and I could barely get 2 bots to work--and even then, they were slower than normal. Now, I'm back to at least 6 bots running almost flawlessly and at about 70% CPU capacity again.