Very low frame rates when rendering to textures

For a simulator we want to use multiple projectors to project to the inner side of a dome.
The first step would be to render the world to 4 textures. But when doing so, the performance is already far below what we need.

I attached a UE4 projects that has nothing in the world except a CameraRender object that takes care for the rendering to 4 textures.
These textures are big indeed. But on my labtop the frame rate drops to 16 fps! Without the CameraRender object it is 120 fps (and probably more than that). The labtop has a K3100M graphics chip, so I do not expect very much out of this. But I did not expect this large fps drop. A system with a GTX980 gives similar drops.

What am I doing wrong? Can I somehow improve this? We really would like to use Unreal for this project, but with this fps drop it will probably be impossible.

Any help to improve this is very welcome.

(I asked this on the forum too. Sorry for this. But I could not attach the project there. And I think this is a more suitable place for this question anyway)