Hmm, I had a similar problem like that.
I use Photoshop to draw, and usually if I plug in my tablet with Photoshop open the paintbrush tool would lag a bit. Although the problem does go away if I close and reopen Photoshop. So uh, I guess I would try plugging in your tablet before opening up SAI. If doesn't fix it, then I dunno what else to try, sorry. :ohdear: