There's more to it than that.. the 1 series processes AF and AE information faster and probably more often, things like iTR need some umph as well.It's like . . . and bear in mind, I'm a 1DX-series spectator, but . . . the thing we hear over and over is that the processors in the 1DX line have to be "so powerful" in order to support the "massive amount of data" the camera is handling, and yet, the DIGIC X processor is pushing fewer MP/sec than a processor in a camera priced under 1K.
RAW video requires a massive amount of data, as does oversampled 4K video.
consider the 1DX Mark II has DIGIC 4, and two DIGIC 6+'s and that's replaced by one DIGIC X and far FAR FAR less power drain with around 2x or more shots per battery, is freaking impressive.
Also MP/sec isn't everything. there's a top end to FPS with mechanical mirror assemblies and there's a difference between how hard it is to make a full frame sensor move data as quickly as an APS-C sensor.