How to get a low latency copy of rendered image from UE4?

I would like to get a copy of RGBA data from the rendered scene from UE4 to send to shared memory using C++. The driving factor for this is to be able to overlay the scene from UE4 on a live video feed for augmented reality applications. ReadPixels seems incredibly slow. Are there any suggestions for how to do this? Or, are there any example projects or tutorials attempting somethings similar?

Wouldn’t it be easier to do it the other way around and just integrate the video feed into your unreal project? Or are there specific reasons why you can’t do that?

Thank you for your quick reply. Unfortunately, I cannot do it the other way around but that is a great suggestion!