Could it be that using the camera via Vuforia limits the framerate to 30fps (Like mixed reality capture)?
After unloading the vuforial scene the framerate doesn’t go back up again though. Are there additional steps necessary to tear down vuforia, or disabling capping?
this seems to be a problem since 2012:
https://developer.vuforia.com/forum/unity-extension-technical-discussion/fps-drop-empty-scene-after-scene-arcamera-android