[OnACoffeeBreak]

https://www.jedec.org/sites/default/files/docs/JESD84-B451.p...

You can use http://bugmenot.com/view/jedec.org to get free-to-use credentials.

The section in point is "A.6.1Bus initialization".

I'd say that if your MMC works fine going full-speed out of the gate and the IC on the board supports this speed, then you unlikely to encounter any issues.

[eerimoq]

Thank you very much. Very helpful answer. I'll continue to use 52 MHz until I encounter problems (if any).