Inconsistent FPS Hiccups only on first run

Have a read through this: 'Why is my ‘Async’ load still causing frame drops?' https://developer.oculus.com/blog/avoiding-hitches-when-loading-scenes-in-unity/

Lots of in-depth advice that may explain what's going on

FIXED!!!! I finally fixed the FPS hiccups and it was not at all what I would have expected. The profiler was giving me a clue about a LogToConsole taking a very long time. I always assumed it was just a side effect and that the root cause was elsewhere as there was literally nothing written to the Console of Editor Log. Today though after 8 hours of messing with it all, I decided to turn off the stack trace which I noticed was on. That was it...all issues are gone and it runs butter smooth. Just to be sure, I turned it back on to full logging and the FPS hiccups returned. Oh well, glad I solved it.

Thanks for sharing that. I'd always heard Debug.Log messages were had on performance but never realised just how much. Great to hear you've got things sorted.

SOLVED:  You all will not believe the issue especially since I could find any mention of this anywhere. It was the skyboxes. I had skyboxes in many scenes (due to duplicating some of them during their creation) and while they do not change the sky in the Player or on the Quest when you async load the scene (which I am really unsure why) they must still have an impact in the load process, especially for heavier scenes that are already pushing the Quest. To fix, I "Set Active Scene" for each scene and then deleted it's skybox as I already set them elsewhere with code in strategic spots in my title. Once I did that all went back to perfect 72FPS without a single hiccup.  

This explains why creating a new scene and copying the contents of an offending scene fixed the issue as a new scene has the default skybox which is a much lighter skybox than my others ones. This is also why when I created that fresh scene another scene would start acting up as the 2 scene shared the same skybox and hence when I fix one, the next one after is now the first to load that skybox in memory.  No wonder I felt like I was playing Whack-A-Mole!  GOSH...this was a hard one to detect. I spent many hours pouring over the various anlysis tools and performing untold trial and error builds, but the problem was very evasive especially since you never see the sky change.  I cannot believe it was the Skybox for all these months. I encourage you to even delete the default skybox on a new async scene as even they have a small impact. I hope this solution helps others and prevents

Again, very much appreciate you taking the time to let others know about your findings.
When you say "copying the contents of an offending scene fixed the issue as a new scene has the default skybox which is a much lighter skybox than my others ones", can you clarify what the difference was between the default skybox material (which uses Unity's Skybox/Procedural shader) and your own skybox material. Where you using Cubemap, or a custom shader?