After hooking up eight LED matrices in series, I could see that the LedControl library wasnt up to the task. After digging through the source code I realized what a peice of junk it was.
I had to do better myself. And did. I even ended up writing my new routine in assembler. Better optimized than even the code gcc managed to produce.
Source code in the form of an Atmel Studio project,
is available here: http://joonicks.eu/arduino/LedControl.zip