Pardon the analogy, but this is like asking why traffic is slow on a Friday afternoon. Could be dozens if not hundreds of possible explanations.
Please give us more specific information about your system hardware, how it is set up, what versions of OS X, FCE and QT are on your Mac, if you updated/upgraded any software recently, if this slowdown is a change in your system behavior recently, what you are doing when FCE appears slow, what format/codec video you are working with, etc. Details will make all the difference.