Heya,
Suggestions:
-reduce the amount of actors active
-depending on your game, try changing the physics settings , or disable physics for actors that don't need it
-inspect your codes for performance drainers, more specifically these - always, collisions, timers
-test on a different computer, could be something wrong with the flash application
-lower the screen size for flash
Last thing you can do is to debug it by disabling/removing codes/actors in order to isolate the cause of the lag.
I haven't been around for a long time so if something is inaccurate please let me know.