The Rubble and NGR have the same size mirror but they are constructed differently. The former is not sharp over the entire field of view, the latter is - that coupled with a much, much larger camera (nearly 300MP) lets it record images at least equal to to HST but over a very wide field of view.

As to speed, it is not just the camera view that makes NGR faster but also physical factors such as not having to wait for earth to get out of the way.