I created this effect a while back for Unity in C#, and I too have been trying to replicate it for UE4. Unfortunately, no success here. There’s a few things you need for it to work and most of them don’t seem to be available through blueprint. Altering the projection matrix definitely seems impossible in blueprint. A few other issues I quickly ran into were dynamically creating render textures, and dealing with differing camera aspect ratios at run time. As for the visual lag, it’s got to be an issue with when things tick. I would guess that tick is called after each frame is rendered, adding 1 frame of lag at least, and then I’m guessing capture actors are delayed in some way, adding more frames. Really not a clue how to deal with this as I don’t really know when Unreal calls it’s various functions and in what order.
Sorry to say that I don’t have a clue where most of these things are in C++. I’m happy to share my C# code from Unity, but I don’t think it’ll help you much. Still, something I would love to see in Unreal.