Now we see through a glass, darkly...
- Apr 5, 2016
I wonder if this is necessarily true. The Canon M6 II so far is the only Canon body to feature a crop mode with increased FPS. You can either shoot 14 FPS 32.5 MP or 30 FPS 18 MP cropped to the image center.
Those two modes have basically identical throughput, as the bit depth also drops with the crop mode. The throughput is 60 % of what they R5 archives at 45 MP 20 FPS.
So if they achieve that speedup in the crop mode by actually reading the full width and just skipping some rows at the top and bottom, that implies the M6 II actually manages 91 % of the R5's throughput internally and its uncropped 14 FPS mode could be much faster if Canon allowed it.
(9/16) / (2/3) * (32.5 MP * 30 FPS) / (45 MP * 20 FPS) ~ 0.91
If instead they actually read just the cropped section and therefore reduce the number of required read operations, that at least explains nicely why the uncropped 14 FPS mode and cropped 30 FPS mode line up so nicely in throughput, and why the numbers are what they are.
Or it could be that readout speed is no longer the bottleneck it once was and some point further down the processing to output chain is...
Just because the sensor must be read out all the way across each line does not mean all of that data has to be processed all the way to the end of the processing chain before being cropped out. It can be discarded before demosaicing. Maybe immediately after ADC.