Just wanted to report that this is happening. On 4.14, using the Vive, you will notice a massive framedrop when using a render to target texture (scene capture component), even if the resolution of said texture is very small (I tested with 80x80, and it cut the framerate by half).
This can be replicated by just adding some scene capture components to a scene and running the game in VR mode with the Vive.