Movie clip slows down

I have a movie clip consisting of movieclip images fading in and out in a kind of slideshow.
What I’ve noticed is that after a certain number of frames, the tweening becomes jerky and really slow. So I split it up into a few separate movie clips and this seemed to help.
BUT, when I view the movie on an older machine with a slightly slower connection, the whole thing is so s l o w and really jerky.
I’ve seen other websites do this same kind of fadein/fadeout transitioning and it is not slow and jerky on the older machine.
What am I doing wrong?

Depends how complex the transitions are. Could you post a sample online?

Upping the frame rate might help, but it could just be that your tweens are causing a major memory hog at certain points. You may want to consider re-thinking your transitions. Doing them in AS will certainly some of your issues.

The transition are simple motion tweens on the alpha value (0-100 and 100-0)
If I use AS will this be better?