Is it possible to obtain RGB data of final result with little FPS loss

I am trying to reimplement my old application using UE4. One of the biggest concern is that: can I obtain final RGB data from rendered screen in realtime.

I used to be a developer on Irrlicht. In that application, It is very tricky to obtain the rendered screen’s RGB data. Actually, I lock the back screen instead of the front one, which is not recommanded (but works). And lock front screen will cause FPS drops dramatically. (BTW, the RGB data is pushed to FFMPEG to get video stream.)

I want to ask if there is a support in UE4 that allows me to read final rendering result (RGB data) with little FPS loss?

Thank in advance.