SPI can be blazing fast. A colleague of mine recently implemented some memory interface in the range of two/three digit MBits per second. Intuitively I would say bit banging an 8 bit parallel data input using a microcontroller is slower than using high-speed SPI. Just polling the 8 bits from the pins will take ages...Of course I am assuming you're using a uc and not a FPGA. You need to be precise, just because you can read data faster doesn't mean you can process it faster. I'd be a bit shocked to hear that an accelerometer could produce more data than a standard uc SPI can take...but then, I also have no experience. So I'm also curious what you are planning to do!