I'm working with Unity (v2018.4) and Mozilla WebVR Assets. Building a project and exporting it to WebVR works fine. No problems occur while running it on a Rift (via Chrome or Firefox). However, running it in Oculus Browser or Firefox Reality Browser (on Oculus Quest) will result in a very laggy/jittery camera movement. Hands are fine, but everything else is not. This is not dependent on the scene itself. The issue can be reproduced by simply open the 'Mozilla WebVR Assets' example which I can't post as link: mozilla.github.io/unity-webvr-export/
I'm struggling with the same issue. I've tried exporting from 4 different versions of unity and stripping out all graphic enhancements. The hmd movement jitter is not a problem using Ouculs Link.