At anytime, click OPTIONS | DISK tab and Clear Unused Cache Files. Do this before starting a new project.
Answer a few questions to help zero in on the issue...
Click the three-lined menu at the top-left of the VP screen, then click FILE | HELP | ABOUT. What version does it list and does it say "64-bit?"
Try this release. For licensed users upgrades are free for up to six months from purchase date. After that, VP will continue to fully function but a fee will be required in order to register the newest. Retain your old install file and registration info. Let us know if caching is quicker.
On average, what's the format (mp4, MOV, AVI?) of the source files and how large are they?
How long is the project?
In general, don't load source files that won't be used in the project and try to minimize the number of tracks.