As I mentioned in one of my previous posts, canon drops the bit rate and/or dynamic range in favour of FPS and fast readout.
Basically with normal sensor you cannot readout all the data quick enough to avoid rolling shutter and shoot at higher frame rate, so they just read less of it
This is less of an issue with a stacked sensor in R5ii but on a non-stacked sensor like R6II or original R5 its massive hit to IQ.
I would like to see the results on R6iii when its up but I feel it'll be more like R6ii, may be slightly better but probably not much more if its not even partially stacked.