progressive slowdown

Making a vertical shooter, but having progressive slowdown as level progresses. No more than 200 objects on screen at any given time.
Destroying objects as they leave the screen. Set res at 800 x 600,
65536 colours (256 looks too crappy). I've got a decent video card. What other strategies should I adopt to prevent progressive slowdown?

Progressive slowdowns may be due to undeleted objects. Is the number of objects stable in your debugger window? (I'm not talking about the number of objects on screen) If not, try to see what objects are not deleted and why.