Lots of factors involved. If your computer is fast enough to run 5 copies of WoW with no "video lag" (poor video performance, poor framerates), then it will be faster to run 5 copies on the same PC. The only thing that changes by putting them on separate PCs is you now have slightly more network lag (not even noticeable to the human brain I'm sure) added in.

The only time 5 PCs will become better IMO is if you are having slowdown due to running out of memory, CPU cycles, or your video card is overloaded. Or if you just want a full screen for each copy of WoW (completely unnecessary IMO but I'm sure opinions vary wildly on that one).

Also there are other disadvantages to using 5 PCs (not using PiP from keyclone, etc).