I never had this problem until I converted to steam. At first, it seems like the game is running fine. I can run around without issues for a minute or so, but then when I'm getting into a dungeon, it's like my character is running in goo. I can't be particularly useful when I'm lagging so badly.
But the thing is, it's not always happening. The other day, when my friend was running me through Snarbolax, things were running just fine. I can't figure out what the issue is. I tried closing out of all browsers, applications and messengers, and everything else I could close out of. Every once in a while it does stabilize, but currently I'm still springing back and forth.
(Also, would I be allowed to make another account to test if it's just an issue with steam rather than an issue with my computer? I don't know how SK is about having alternate accounts.)
Did you always play using Steam, or did you have the standalone install of the game? Steam uses an older version of java (which you can replace), but also has the Steam overlay, which can sometimes cause problems for unknown reasons.