What if combine FPGA speed and flexibility with Arduino simplicity. You get ZPUino. It is a great project that reached stable release 1.0. ZPUino is a soft processor running on FPGA. Probably not the first you say. But it is easy programmable by using modified Arduino IDE.
Processor itself is 32-bit and is capable of running at speed of 100MHz. But most important part that unlikely in regular Arduino you can construct your own peripherals and other hardware using FPGA logic gates and other resources like memory hardware multipliers or other blocks. As we mentioned there is a ZPUino IDE based on Arduino IDE. So practically there is no learning involved… If you have FPGA hardware like Papilio 500 or Spartan3E Starter Kit around give it a try.