Stat gpu - what is composition PreLighting?

Posted this also on the Unreal forum but no answer. Hoping someone here knows more.

I’m looking at ways to improve an Oculus archviz scene. Its a house; an interior with surrounding landscape. I set a fixed frame rate of 90 but the fps is 45 most of the time.
Looking at the profiler data, the cpu is waiting 22ms for the gpu. Stat gpu reveals that almost 40% (8ms) is used by ‘composition PreLighting’.

I would like to know what this is and what I should be looking for to improve this ‘composition PreLighting’? I already searched Google, this forum and the Answerhub and no answers to identify this. A new feature maybe?

My guess was ‘dynamic lights’ but I changed those (one lightsource for the sun and one skylight) already to static.
Using 4.15.1 & deferred renderer.

Still looking for more info on ‘Composition Prelighting’. No-one has an idea?

I tested the forward renderer and that gets me to 11 ms again but for now I would prefer to fix this Composition Prelighting and use the deferred renderer.

136466-compositionprelighting.jpg