In general, the drawing tool may not be as responsive to your mouse or tablet as a non-web based application. It's possible that the Flash Player (the plugin in your browser that loads the drawing tool) is lagging a little. This could be due to many reasons - firstly I would make sure that all other applications are closed while you are drawing and you have no applications like music playing on your computer at the same time. You can also try and draw slightly faster to try and get less jagged lines.
Thanks for the reply. I don't think Flash is lagging, although I went and updated it when I read your post. I still have the same problems even after the update. I don't thing it's my computer either, since it's built for RAM-eating programs. I guess it's my tablet. I guess I'll wait until the drawing tool gets a little more advanced. Thanks anyway!