I recently just bought a canon EF 70-200 f2.8 L IS USM to use with my canon R7. The lens works perfectly with the R7 and the adapter and camera but when I switch it over to my Canon R6 and put it on high-speed + the high-speed logo just blinks and when I press the shutter button the shutter goes very slow. I also brought it to a camera store, and they couldn't figure it out. Is this a mechanical issue or is it that the R6 does not work with canon EF len's?
I believe there are some limitations on using certain EF lenses with R cameras, presumably you can find a list of these on the Canon website. I would suggest in your case it may be that having both in body IS and lens IS (and perhaps high speed display) enabled could be slowing things down and preventing high speed burst working properly.
