There are some boards with Pick processors they are very nice not completely open source but faster than the atmega . DUE for me has been a disappointment although the processor is blindingly fast the software and libraries are not ready , in six or eight months to a year ti will be better. I am redesigning my project to use a Leaf labs maple, a compromise, more pins than UNO, nearly the same speed as DUE, still has a arm processor. The big down fall is all of these Some what zippier devices are 3.3v at low current. This requires more attention to detail when designing a circuit. Careful not to smoke the cpu.