You may be right for this application, especially given uart support and/or library support. But a synchronous protocol can be very nice and simple, the clock line eliminates all sorts of timing considerations. For a really simple example ( not I know the full spi or ic2 protocol ) think about clocking out to a shift register.
And yes I have lots of emoticons ( even if I am not sure that I can spell it )
That can be pretty darn quick too. And I have done the I2C thing before. Yeah you have START and READ / WRITE things to fiddle with by the levels and edges, but it can be made to HAUL THE EMAIL so to speak. Nothing "slow" about it unless you have a clock restricted source. Outguns most of the RS232 I've tinkered with but it can haul the mail too at the circuit board level.
The controllers I play with have bits to set up the uarts as SPI and I2C