As stated in another thread, CPU tends to be a big factor too; especially if the processors are single-core, that's gonna hurt.

Overall, WoW is by no means graphics intensive but multiboxing, even split among machines, introduces unique issues, like -always- having other players on screen, extra spell effects/totems/etc, only thing you can do on those is either upgrade or tone down settings + resolution and disables every mod you don't need on it lol