While making my iOS game I have been testing on a nice shiny and newish iTouch 4th generation which everything runs speedily along but also on a 2nd generation iTouch which most of the time runs speedily but at times of heavy load it slows down. The device is just not as powerful as its more modern versions. When the device is slowing down, so is my game.
To keep the game running at the same speed, or appear to, I can use MMF's built in movements and tick the timer based movement option for each object. When the frame rate drops due heavy load, MMF compensates by moving the objects to where they would have been if the frame rate was remaining constant.
Sometimes you may want to move objects yourself. This example shows you how I have done this in a similar way to MMF's timer based movement.
Tap the screen to change the frame rate between 60 and 30 and the object moving with our timer based movement keeps moving at a constant speed. When you employ this technique on your objects, everything will keep moving at the speed you defined even if a few frames are dropped during heavy load.
This is one way to achieve this result which has been working well for me.