Could be due to a lot of things. Please check/answer the following:
-Which VR device do you use?
-Do you have the latest drivers installed?
-Rebooted your PC? (Sounds dumb but often helps)
-Anything else running that costs performance? (Several UE4 instances / other game engines / games / crypto-mining, …)
-Often a restart of UE4 helps too
If all that doesn’t help check if the performance is good in an existing VR game, if so it could be your scene/game.
-Did you try a completely new level?
-Any fancy calculations going on? (Though they should run on the CPU)
-Check the GPU Profiler - you should be under 11ms per frame (-> 90fps for Vive/Oculus), there you can see what costs you performance
General tips and tricks:
-Use low-poly meshes and LODs (UE4 can generate them for you)
-Use normal maps for details (though they can be strange in VR when up-close)
-Use static lights and objects whenever possible
-Use instanced stereo rendering
-Use forward rendering and MSAA
-Limit the amount of objects in your scene, since VR uses a lot of performance
Hope I could help!