Persistent FPS Drops in Unity VR APK Despite Optimization Efforts

You mentioned using the Unity profiler but you only mention doing a build after, so I assumed that the profiler is running in play mode, in the editor? The first thing to try is to do a developer build of your APK and let it run wired to the PC so that it connects to the profiler via ADB - while the app is running on the device. And then see where you bottleneck lies. Are you CPU or GPU bound? What’s holding up the render thread in those problematic frames. Also keep an eye on the console or use logcat. Are you spewing out a bunch of log statements? Errors are obviously a problem but even Debug.Log can have a performance impact

TLDR. Do on device profiling