Hi jjaken
Another thing you can try to improve the performance of the SPI speed is to change the SPI clock divider, by default if set to SPI_CLOCK_DIV4 which correspond to 4MHz. You can increase the speed to 8.33MHz by using in the setup:
setClockDivider(SPI_CLOCK_DIV2);
If you notice an improvement but that is not enough you can change the source code to get higher speeds. The library is located in C:\Arduino-1.5.3\hardware\arduino\x86\libraries\SPI, in the file SPI.cpp change line 129 from maxSpeedHz = SPI_CLK_DEFAULT_HZ << 1; to maxSpeedHz = SPI_CLK_DEFAULT_HZ << 2; in order to get 12.5MHz, or to maxSpeedHz = SPI_CLK_DEFAULT_HZ << 3; to get 25MHZ, but I’m not sure if this changes will affect anything else. I hope you find this useful.
Regards,
JPMontero_Intel