Bruce and Cville are right on - the chief decision of rendering speed mostly relies on the capabilities of your computer, mainly the processor. The amount of RAM memory calso plays a factor but not near the as much as the proesssing speed.
Different types of software do pose different rendering strategies, but the solidness of your system is the primary dictator. Video software that embraces the quality of its graphics more than anything else GENERALLY gives off more lag. In some cases, these are merely "consumer gimmics" for an intended audience.
Like I mentioned above, this isn't always the case. Cyberlink PowerDirector 7 (and even more so with 8) run very smoothly. Being software that's aimed at consumers, frankly, I've been very impressed. It's a great combination of the right graphics, features, and degree of stability.
Nice going, Cyberlink