For better or worse, we're flirting with the edges of Flash sanity. So, if you experience low frame rate, open your Prefs and turn Particle Effects to low or off, turn Day/Night cycles off, and turn Weather off.
Sound and floating Combat Text should not heavily impact frame rates, so you can probably leave those be. You can also try a more direct link to the .swf file. Finally, closing other graphics intensive apps you may have open in the background,
including other web pages, will help.