This is completely normal. There is a lot more data that needs to be processed with your space ship, than with a sphere. Unfortunately, there really isn't a fix for it besides upgrading your computer, which doesn't guarantee a better experience.
What you should do, is right click on the time line, and click playblast. This will go through each frame, and create a movie of your entire animation. This will ensure a smooth playback speed.
Imagination is more important than knowledge.