When framerate goes down, it's typically a result of 'too much going on at once' which requires more calculations than your computer can deliver at the given frame(rate).
I suspect too much detail in the vector drawings, either in the background or the foreground object - or both, is the reason here.
- In Flash you have tools for counting the number of vectors, which will give you an idea, and also tools to opitimize the drawing by removing very small or even double vector points. The process is automatic, but based on your settings. You will probably need to experiment a bit. Remember that a spinning wheel needs a lot less drawing detail (in the spokes for instance) than a wheel that is still. This goes for any moving object. In Flash you can have a 'simple wheel' drawing for the spinning motion, and a 'pretty wheel' for start and stop. This concept of simplification has been used in animation for about 100 years ;)
Moving gradients are also harder to play back than moving areas of solid colour.
- If the moving background is just a simple pan / scroll, you could optimize this by using the .cacheAsBitmap function. This will take a sort of 'screenshot' of your drawing, and this is much easier to move smoothly than hundreds or thousands of vectors... Naturally, you can't have any details moving in this perticular MovieClip. It will probably not work well on a wheel's moving spokes, but it may actually work on the bike's body, if it's still (i.e. just moving across).
Not everything that can be counted counts, and not everything that counts can be counted.