I could repeat the crude test with AF and IS off but even if the frame rate came up I don't think most folks would be pleased to shoot that lens with only manual focus and no IS just to achieve 12 fps.

The copy of the Version I 500mm that my son has is a really really good one ... it is razor sharp on the 1D MKIV but it is actually heavier than my version II 600mm. He is creating some great images with this combo and because he is 25 years younger than me and in great physical shape the weight is no issue for him. I'm pretty sure he would not like it if he had to use manual focus all the time and no IS.

Anyone who owns a version I 500 and is contemplating an R5 maybe should consider trading for an RF100-500 or a newer version 500 or 600mm.

My 600mm II works great on the R5. At Laguna Seca I was shooting 2500-3000 images daily with the battery grip and no battery changes.

Another thought .... perhaps the upcoming R3 and/or R1 will achieve max shutter speed with the old 500. Looks like these will be similar to 1D bodies with the bigger/more powerful battery.