Yes, just that question.
-jim lee
Yes
It's a slower interface than what most devices use. It's kind of a backward-compatibility thing which allows us to do great stuff with Arduinos even though the megabits-per-second is a trillionth of what the card is capable of.
Yes, it's a feature of microcontroller chip.
With 16 MHz processor, can do 8 Mbit/second transfers (17 clocks per byte minimum, almost 1 byte/microsecond).
Many slaves as ADC and DACs cannot be updated that fast, need to read datasheets.
Many are perfectly happy with the default 4 MHz setting (such as shift register), others need the clock slowed to 2 MHz or 1 MHz.
I'm referring to digital cameras that get transfer speeds up to hundreds of megabits per second to the same card that we're struggling to achieve thousands of bits.