Runescape might seem lightweight but it's a fairly CPU heavy game. You can get away with running a few clients (maybe up to 2) on an i3 processor comfortably with runemate before it starts to give you trouble if that gives you an idea. The pi is just not powerful enough to play the game && run runemate together. At best, you might be able to play a very low quality version of the game with CPU set on max and all else set to low graphics.
No, the single most complex thing about runemate is the communication between client and game. The rest is only marginally affecting workload in comparison.
