Permanent FPS Drop After Using High Resolution Screenshot With Planar Reflections

After taking a high res screenshot with a planar reflection actor in the scene the engine will experience a permanent performance hit relative to the resolution of the screenshot. For example if you set it to 1x resolution the hit wont be that noticeable if you set it to 4x the hit will be much greater.

I could only repro this in full screen mode in 4.15 however, in 4.14 I was able to repro this in the normal viewport.

1. Open a blank project
2. Enable Global Clip Plane for planar reflections and restart
3. Add a planar reflection actor to default scene
4. Show FPS (open console and type Stat FPS)
5. Open High Resolution Screenshot tool set to 4x resolution
6. Make viewport full screen and take screenshot

RESULT
The FPS will drop significantly (120 to 60 in my case) and stay that way from that point forward.

EXPECTED
A high resolution screenshot is taken and there is a small hitch while this is being done.The engine then recovers FPS after this operation is done.