Flash8 :: 'clouds' Motion Tween Is Slowing Down Entire Project

I'm making a very simple Flash game which is just somewhat of a training project for myself. All it involves is clicking a button which takes away a random bit of health from an opponent, and I'm quite proud of myself for figuring out how to subtract the random number hit away from his total health and show the resulting number above his head. (lol i fail)But my problem has nothing to do with actionscript, it's my cloud layer which I've motion tweened across 150 frames inside the "cloud" movieclip. All the clouds do is slowly move across the stage (I created the cloud movieclip to where it doesn't jump back to it's original form when the 150 frames are done, so it looks like one continuous cloud scene). Whenever I test the project, it goes at about 5 fps, which is very slow (supposed to be at 30 fps).I know it's the clouds that caused the laggyness because I temporarily removed them and it goes just like normal.