One possibility is that it's not playing back at full speed because the clip is just really big and your system can't play it back with all the particles at full speed. Does it play half speed once you render the project out, or only when played back in particleIllusion?
It could also be a difference between the frame rate of your original clip and your project in particleIllusion. Double-check in the project settings that they are the same, though you should be given a warning when you are importing the video about a difference.
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Elvis Deane!
The Apprentice Magician's Guide to particleIllusion
Wilbur of Wumbaberry
