Hi
It may be that you have set up a high preview resolution. Even using the latest version of VP (4.58) which uses multiple threads to generate the preview image and thumbnails I find that "Building Preview" message will appear if the resolution is set too high.. I usually set the preview resolution at 768 x 432 pixels and this usually prevents the message appearing.
So...Open VP.
Load your project.
Click Tools/Options/Display
Change the Preset value for preview resolution to the lowest resolution (512 x 288) and see if this cures the problem.
Note that VP 4.05 doesn't use the latest algorithms to generate the preview screen or thumbnails or transfer them to the cache so there may not be much of a change but try anyway.
You could also try clearing the cache completely and reloading the project. (The first reload may take longer with the cache empty, but subsequent reloads should be faster. A lot of unused files in the cache from previous work can slow things down.
Nat