Unfortunately, the SAMD processors are not as much faster than an AVR as you might hope.
While the clock rate is higher, the "special instructions" for writing to pins are gone, and it takes the ARM 3 or four instructions to do what would have been a single instruction on AVR. I did some experiments recently, and found digitalWrite() to be about 3x faster than an AVR (310kHz max toggle speed.) Best all-out-effort pin toggle speed was also about 3x of an AVR (12MHz), but it's a bit less "general" in some sense. You'd have to carefully craft and benchmark specific code cases. (For example, the 12MHz toggle code on ARM uses two registers, while the 4MHz AVR code uses none. And there are only (sort-of) 8 registers on ARM CM0.)
https://forums.adafruit.com/viewtopic.php?f=57&t=133497#p668317