There's a few things you can do.
- Update your Java to the latest version.
- Try tinkering with your RS settings till you get a higher FPS.
- Update your graphics drivers to the latest version.
- Do a disk clean up + disk defrag.
- Try running through the RS Client as some say they receive slightly less lag through that.
Ultimately, it's the game that is causing the lag, Jagex have stated that this should be fixed soon.