Right, unfortunately due to some whacky issue with the “new year” date, it appears that the Editor discarded much of the existing Derived Data Cache content 
So, you can either wait it out now and eventually it’ll finish loading after probably an hour (after that it’ll be fast), or later today we’ll be uploading a completely rebuilt Derived Data Cache fileset to the Epic Launcher so all future loads will be fast.
FYI: The Derived Data Cache is essentially a database of “intermediate” under-the-hood data of the game content which the Editor uses for real-time editing (different from the actual UAsset source files themselves). If it is missing or outdated or clobbered (as happened with this new year), the Editor tries to rebuild it dynamically and… hence the wait 
Cheers,