When you experience a performance drop drown, that means the video memory of the Graphics Card (storing the images) is full. The Display Driver then swaps some images from video memory to main memory. This process slows down the playback.

Playing a movie allocates some video memory. Playing another one at the same time in another application (screensaver, exe, composer, etc) will require some video memory too. If no more memory is available, the playback will slow down.

Possible solutions for reducing the amount of video memory required by your Movie are:

• Reduce the size of the largest images: in the View menu, click View Images Library, and click the Memory tab to sort by Memory. You should see how much memory takes each image on the card.
  Close the Images Library dialog box. Then select the image you wish to optimize, press F7 to open the Images Optimizer. Reduce the size of the image by choosing a new memory allocation strategy. Notice that it may produce less precise images.
• Change the Zoom-Out smoothing of some images (in the Images Optimizer dialog box) from Nice or Perfect to None or Simple. This will affect the quality of the smoothing but will save some memory.
• Reduce the precision of the titles (try 48 points instead of 72 for instance).
• Reduce the overall number of images used by the movie.
• Change the screen resolution or screen depth.