Pushing down the frame rate on slaves is the #1 thing you can do in my experience to improve system performance. I have a reasonably powerful system and I still need to do this.

There are two different console commands to set frame rate, /maxfps and /maxfpsbk. /maxfpsbk is the more important of these. It limits the frame rate for WoW windows that are in the background, which will be your slave windows most of the time. I recommend setting this to 5 or so. It does not impact how the UI processes your spells/commands.

Setting other visual effect setting very low on the slaves will help as well, although you may be ultimately limited by your system from running a full 5.

In relative order of importance based on my informal testing:

1. Screen size/resolution - small
2. Aliasing - as low as possible
3. View distance - turn it down

Then, other visual settings like spell detail, etc.