Are you sure it's their end and not yours? I haven't played RS in quite some time, but I do know terminology. Lag could be on either their end or yours (or somewhere in between, which is another beast), so be sure to cross off your end first. How is your latency, or more specifically, latency to one of the servers you're trying to play on?
Try using Pingtest
to a server near where you're connecting. If you get a low score due to high ping or high jitter, it's quite likely that your connection is the problem. If you end up getting a good score, then you can start assuming it's Jagex's issue.
What kind of games do you like? There's *tons* of stuff on Steam, including plenty of free stuff. Do you like FPS games? Puzzle? Or do you not care as long as the price is right?