Cool. I have some EF lenses. My best one is the Tamron 24-70 f/2.8. I have it on my M6 mk II with a speedbooster,. I also have a mount adapter but I usually just use it for my EF-S 24mm f/2.8 (pancake).
I have the R7 with EF 500 F4 L is (mk1) and EF 100-400 L is ii. I use the 1.4x TC iii and a Metabones 0.71x speedbooster. Everything works well. Mainly for birds and wildlife. Total game changer from the 1D3 and 7D I had before.
I shoot an EF 600mm f4 IS II. I use the 1.4X and 2.0X EF Extender III's. That combo is known to work very well. I used to shoot it with a 5D IV. When I moved to Mirrorless I was pleased to find it works as well, if not a little better with my R3.
I recently upgraded from my circa 2008 50D I used with an EF-S 10-22mm, 24-70mm 2.8 L and 70-200mm 2.8 L + 2x converter to an R6MkII with a RF 100-500mm and RF800mm f/11. I use the RF lenses on the R6mkII for stills but find it much more convenient to use the EF lenses with a VND EF-RF converter and my EF lenses when shooting CLOG3 video at 1/50th and ISO800, so much so I bought a second R6mkII body went Canon recently dropped the price to $1999 instead of spending $500 for ND filters for the RF lenses. I can carry both bodies, one ready for stills and the other for video 😊
Currently using R6 with 500mm f4 version 1 and although the AF is slowly than native RF lenses the image quality and subject separation is immaculate. I usually use the 1.4x III as well but sometimes no TC looks so sharp it’s worth it when capturing larger mammals and birds
The EF 500mm f/4 MkI on my EOS 5D MK4 and EOS 1D X MK2 was fast and sharp. On my EOS R5 only 7 shoots per sec, soft images and the focus preset wasn't working. The EF 500mm f/4 MK2 have 12 frames/ sec, sharp images and the focus preset is working on the EOS R5!
I'm using the R3 + 400mm F2.8 IS II + 1.4x III and I haven't really noticed any differences in af speeds at all, otherwise pretty much sharp wide open at f4 and 560mm
