You shouldnt have problems really unless you are in a main city.

Just bump your graphics down on all the wow accounts that you have to the minimum. If you are running 4 accounts on one computer you are not here for the looks anyways.

Just bump the graphics down all the way close any other background programs.

If you are still running slow after this partition your windows to its own partition and partition your wow to their own partitions.

I run 4 on my laptop and the only problems I get is loading into a new zone or loading into a city.