Hey, as you can read here: https://steemit.com/steemit/@musicbox/how-to-dtube-having-problems-login-in-dont-know-how-to-use-it
DTube does NOT reencode your videos. It stores and plays them in the format you upload them.
A bit downwards in the comments you can see some suggestions,
e.g. upload your videos in MP4 or WebM.

Hope this helps!