Interesting, as I find the 40 STM to be considerably slower than the 50mm STM. I've also used it some recently on an 80D on a motorized gimbal for video work, and ended up switching to the 24mm STM lens because the 40 STM was both slow to make focus transitions and also much louder in operation.
Well that's definitely interesting. Now that I'm back home I just gave it another try. I had admittedly never tried the 24mm STM, since I have no intention of using it on the M5, having the EF-M 22mm already. But I gave it a try here.
In a nutshell:
- there is definitely no significant difference for me in focusing speed between the 40mm STM and the 50mm STM. If anything, I would say that the 40mm is a hair faster. But again, nothing significant.
- the 24mm STM is the fastest of the lot, but the difference is not night and day.
- the 40mm STM is indeed louder (I usually don't pay too much attention to that, since I don't do video).
Overall, as I mentioned, I find the 40mm to be rather well suited to the M5, just as much as the 50mm. At least in terms of focusing speed, I see no difference.