You only need a 45 or 60 mB/s card because as you stated the writer is only capable of 40mB/s.
On the other hand that h264 video was probably in megabits not megabytes and hence it was 17.5 megabytes per second.
Yep, thats why I put the 'mb' in lower case! Me knows the diff!! Me use to be an Electrical Engineer. Me are used to bits/nibbles/bytes/words/double words/etc. Me be cognizant of the diff. Me prefers 'MB or mb' rather than 'mB or mb', for the emphasis, as well that is what is shown on ML screens - 'MB/s'.
The statement of compressed video is that it will NEVER likely be able to be written at the speed of raw or M SD writer, as the H.264 encoding does take time and the encoder chip is likely processing limited, or I would see a higher bps, wouldn't I?
Also, I can set CBR to higher than 2.9x (4x is where I quit) with my card, just the output NEVER gets higher than about 140mb/s (saw peaks of 150+mb/s), so VERY likely processor/encoder/bus speed limited.
Also, the ultimate speed of compressed seems to a degree dependent on ISO, maybe - limited use/testing of this.
PS: It is going to be interesting when UHS-II (240MB/s writes) SD writers are in cams, and also when H.265 encoders are in cams, then the Digic engines (dual/triple Digics?) will have some REAL work to do & will be so capable !!
BTW, FYI: Canon works (has work done by?) with Texas Instruments (TI) on their Digic processors - Nikon has FujiFilm do their h/w work & firmware work!