Just received the Canon R with the adapter (no ring) rental. Here are some observations on different lenses.
Canon EF 24-70, EFS 10-22, EFS 18-135, EF 100-400 (first gen) all work great, such as fast autofocus.
Canon EF 100-400 (first gen) with extender EF 2x II has extremely slow autofocus. I...