Thanks for the responses. I'll try a few different things when I make a new moviestorm video and see if I can get it to work smoother.
The video I had the largest slowdown on, I had five human characters and about a dozen zombie characters, lots of action and dialogue, and about five different sets going, and many camera angles, it was over 10 minutes long, so I can see how that one had major slowdown.
In my latest one, I had a lot of special effects going at once, despite it being a single set, so that may have contributed to it.