I was experiencing the 'slow motion' constantly until I made these tweaks.
Note: If any component of your computer is below the minimum requirements OR is not supported, the best fix is to upgrade.
With that being said here are a few things you can try:
Let's get the obvious out of the way,
Change all possible in-game settings to the lowest possible.
Resolution: 800 x 600
[ ] Letterbox
[ ] Vertical Sync
[x] Max Foreground FPS - 60
[x] Max Background FPS - 1
Texture Quality: Low
Shadow Quality: Off
Clutter Density: Off
[ ] Anti-Aliasing
[x] Low FX
If your setting look anything close to this, and you are still having problems playing the game decently then I have a couple tweaks for you.
Locate your D3Prefs.txt file.
For XP users the path will be something like
C:\Documents and Settings\User Name\My Documents\Diablo III
I've found that changing the following options can greatly improve performance.
If those settings are not set to the above, then changing them MAY improve performance.
The game might look like !@#$, but at least you can play - hopefully xD
Note: There is a good chance you will still lag in select areas. Some areas that have a lot of effects going on, water or water falling, fire, haze or fog, or sand storms may still cause you FPS drops.
I hope these helped at least a little, back to D3 for me!