First of all, this isn't my guide, but a repost from a separate site. Anyways, this is useful if you want to bot on D3, but your computer can't really handle it, or you just want to bot on multiple accounts. Anyways, here it goes.
1) Copy this into a new .txt file and name it "D3Prefs.txt": D3Prefs.txt - Pastebin.com
2) Navigate to: C:\Users\%username%\Documents\Diablo III\
3) Replace the D3Prefs.txt with the one you just made.
4) Start Diablo III.
5) Enjoy.
My results with the tweaked D3Prefs:
Before (%) - After (%)
CPU: 85-95 - 35-40
GPU: 50-60 - 25-30
Graphics are definitely not the best, but it isn't like you're playing anyways.
EDIT: From CHandest: Also you might want to change hardwareclass to "1"
And from sssniper86: I'd like to add that you might need to one line to this file:
LockCursorInFullscreenWindowed "0"
And modify number in
PCIDevice "4544"
to correspond to the one that is in your default D3Prefs.txt.
Make sure to rep them both if they helped as well