Render subsection of frame I'm hoping to be able to render frames of an animation where I render slices that are set to different points in the animation. This effect could easily be achieved in after effects but the render time in Blender would be drastically more so ideally I'd like to cut it down. (Have lots of slices and cameras to merge into single images)

Is there a way to have the animation at frame 1 render the top 50% (or any number of pixels) then move the animate to frame 50 and complete the bottom 50% of image. This would become frame 1 of the rendered footage. 

Same then start the next frame this time with frame 2 in the top 50% and frame 51 in the bottom 50%. And so on until we have the completed animation with bottom half offset by 50 frames.

Any advice would be appreciated so much.
Blender will only render the scene as it exists in one specific frame, meaning you can't change frames half way through a render. While you can use the compositor to crop half of the image out, the entire image will be rendered before the crop happens so no time will be saved.

The only way I can think to reduce the render time is to cut the horizontal resolution in half. That would also mean moving the camera to compensate for the altered resolution, two cameras could be parented to stay in the same relative position so that both renders match up. Camera one can then be used to render frames 1-50 and camera two for 51-100. The two "half" images can then be composited together to get the final result.