They do whatever speed they're rated for there are a wide variety. FPGA stands for Field Programmable Gate Array. It's a network of logic gates which use a special programming language to trip the various networks into forming logic functions that you describe, it's basically programmable hardware; but not in the sense where you write code for it's native instructions to execute, you actually write the entire logic for the instructions themselves, so they're very very flexible. They're able to run complex functions in near realtime, they're physically large compared to the function they may execute though because they're designed for flexibility. They are not easy to learn though.