GPU, then back to CPU... ?

For maximum speed avoid any and all readback to CPU, and where possible, program shaders to do this. There's no way to avoid some significant impact on the FPS by reading back to the CPU.

Where there is something which doesn't translate to the GPU easily, such as image tracking with cv.jit, or FFTs with jit.fft, or other image analysis for parametric control of the Max patch, then you will want to read back. jit.gl.asyncread will probably be your fastest option for readback to the CPU, but it can introduce a frame or so of latency which might be undesirable. I'm unsure of your application. If you are looking to get mean brightness and perform gain control on the image, or something else which needs to have the CPU frame processing in perfect sync with the GPU frames then the potential latency from this asynchronous method isn't appropriate. IN such a case use the method in the image you've posted.

Basically, if you could tell us what you want to do on the CPU vs GPU and why, we could offer you more useful information, but the short of it is don't readback to the CPU unless you have to. If you do have to readback to the CPU, for maximum performance, decrease the resolution where possible, and use jit.gl.asyncread if acceptable.