stephanschulz - I think the thing that gives the high performance are the LED strips which handle PWM themselves.
For example, have a look at http://www.adafruit.com/products/306
which have Arduino libraries for driving the LED strip.
Maple has two SPI peripherals, so it could drive two sets of strips with very low overhead.
Edit: The performance of the mbed they use is pretty close to Maple, i.e. it is the same Cortex-M3 processor, running a bit faster. Hence I think pushing the problem of controlling brightness into the smart LED strips has a much more dramatic effect on the performance of the overall system. Once set, the smart strips will handle PWM for themselves until the brightness needs to change.