Hello mangurian! The Mount Adapter EF-EOS M allows all Canon EF and EF-S lenses to be mounted on the Canon EOS M Digital Camera. It is fully compatible with all lens functions, including image stabilization and autofocus!
"Why would it make anything slower??"
I suppose you need to ask Canon why. All I know is when we were shooting my friends M5 it didn't perform as well with the converter and ef lenses as it did with ef-m lenses. It is slow compared to my DSLR even with ef-m lenses. Most of the time it over shot focus and had to come back with the converter.
IMHO, any time you 'convert' something to something else it often doesn't work as well as it does on the thing it was designed for.
09/26/2023: New firmware updates are available.
08/18/2023: Canon EOS R5 C training series is released.
07/31/2023: New firmware updates are available.
05/31/2023: New firmware updates are available.
05/18/2023: New firmware updates are available.
03/30/2023: New firmware updates are available.