woskars,

Presumably you mean not running smoothly when you are previewing in the editor? This is most likely due to a combination of complexity of your project (complex transitions, multi track layering etc) and an under-powered computer. This may help:

http://www.magix.info/us/what-can-i-do-to-improve-computer-software-performance.knowledge.1097374.html

Additionally you might post here your computer specs and someone, probably John, can give you more specific advice.

Regards,

Carl