Profiler: What is 'RenderViewports' and why is it destroying my performance?

I’m running Unreal 4.20 and I’m currently developing a game for Oculus Go. My profiler shows the main culprit of my poor frame rate is "RenderViewports’ which is taking 87ms. Does anyone know what this? Is there a glossary of profiler processes and terms that exists? If not, why not? This is an absolutely vital part of dev and without understanding everything in the profiler it becomes useless. I’ve searched and only found two other users having the same issue with no answers. Any info would be fantastic, thank you.