Hi
I spend some time to analyse SdSpi.h. I finally ended up to add SdSpi to ucglib (my own graphics lib).
Regarding speed, my results are a litle bit different.
With the standard SPI.h library, results are this:
ILI9341, Arduino Due, HW SPI Text: 12.4 Box: 22.4 Gradient: 16.5
With SdSpi.h and some optimization, i get this:
ILI9341, Arduino Due, HW SdSpi Text: 13.9 Box: 38.0 Gradient: 19.7 with multibyte transfer
My text and gradient speed test do not show much improvement over SPI.h. This means, that the original SPI.h library is already quite good. Main improvement is there in those cases, when multiple bytes need to be transmitted. Here speed is almost doubled.
Oliver