Adding a media track(.mp4/avi) into a level sequence causes the editor to hang when scrubbing the timeline, or enabling/disabling the media track.
1. Create a Sequence and add a camera to it.
2. Key some random movements to the camera track
3. Add a media track and assign a file (.mp4/.avi) media to it
4. scrub the track or enable/disable the media track
Level Sequence Editor scrubs and functions normally
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-193841 in the post.