Correct. The CFe B card slot is definitely not maxed out for write speed. The write speed of the USH-II SD slot could be maxed out though with 45mp raw/20fps. Given that the R3 will have mixed slots again, then it makes some sense for 30mp raw/30fps to enable dual card recording of stills which would be mandatory for this level of camera.You must also be assuming that the bandwidth of the R5 is maxed out.
That need not be true.