The issue I've experienced recently. Whenever my view is completely covered by a static mesh with DefaultLit material on it (I'm standing really close to the wall and looking directly onto it, for example) Frame render time jumps up, even though Draw thread isn't affected. If I look along the wall, so it is in the frame to be sure it's rendered, everything is ok. And it appears gradually the more percent of a screen is covered from about 60% to 100% at the peak. It looks like some kind of Fill rate issue. The only stat I've found to show this is SlateDrawWindowsCommand which is pretty abstract.
As you can see below it's an empty scene with simple skydome (Unlit material) and a wall with default tiled material (any DefaultLit material will do the job) So the wall covers only part of a screen and render time is fine.
And this is example of 100% covered screen. Render thread has dropped significantly (rounded to 22 ms by 'stat raw')
Any ideas? Is there a fill rate cost of some sort?