The only thing that will suck up processor is something that is actually doing something. If the movie clip is looping, but nothing particularly interesting, or complex is being performed, it shouldn’t take up too much power.
The biggest hogs of processor are the following.
a) continuously updated text fields.
b) an object with alpha not 100% moving across an image, or complex vector construct.
c) a complex vector object moving. (complex would be something that contains a few dozen vectors.)
d) an object made up of a image fill, moving.
If any of your movie clips are doing these things, you might consider stoping them while other things are going on. Really it depends upon the user though, so you’d have to test on some older machines to see if your production is sluggish at all.