Wait for caching to finish. The message will vanish and preview will be smooth. How long that takes varies with the the project. It's not new.
Video clips are fully cached when the green line above their thumbnails is complete. 
Stills aren't cached.
The entire Sequence is fully cached when the ribbon under the preview window has changed color... 
Use 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.