I'm using the Adafruit 2.8" TFT too. I got the SPI side working a few days ago, driving it with MRAA I get about 400 ms for a full screen refresh, ie about 2.5 frames per second. My goal is to see if I can get to 30 fps, maybe getting close to 8x out of the 8-bit interface, and 2x by going from 16-bit to 8-bit colors could get me there.
I have kind of low hopes that user space programming/MRAA can get fast enough, but I haven't tried the memmapped calls at all yet. Thanks for the pointers here! Looking at the mmap implementation in MRAA and moving the data bits to continuous Edison pins (looks like Arduino 10-17 map to Edison 40-47 - not in that order though) could allow me to update all 8 pins with two 32-bit writes.
If that's not enough, maybe I'll need to do some kernel hacking and write something like my own "display driver". I haven't done that before, but it sounds like an interesting challenge. I'm pretty fearless on the software side, just have a somewhat limited time.