This issue has gotten logged in our tracker here: Unreal Engine Issues and Bug Tracker (UE-42280)
Look at the ticket for a small list of workarounds that may help the issue.
If you want a quick and easy workaround that seemed to help a lot of people do the following.
- Open the editor
- In the the Content Browser select View Options (eye icon bottom right)
- Uncheck Real Time Thumbnails
This was proposed by a user (IronicParadox).
Cheers,
Ed