It already does cache them to a seperate database file, uvThumbs.db. If all your videos can be successfully rendered, it should be very fast to show all the thumbnails after they are first cached. However, if a thumbnail isn't made for the video since it can't find a compatible codec, it will keep trying to load it every time. This is done so that it will retry to cache files if you install a compatible codec to make the video now work.
This has been fixed a bit in 18.104.22.168, it will still try to recache any invalid videos everytime its started, but it will remember whether they failed or not the first time you view a folder until you close StreetDeck.
No, it shoudln't ever crash. Please send the relevant logs to [email protected] and include the name of the codec giving problems.