18 MHz, actually -- the FPGA clock speed is 73 MHz, but it executes the equivalent of one 6502 clock cycle in four of its clocks.
That being said, this was implemented on a budget-line FPGA from 2006 (XC3S50A - a small Xilinx Spartan-3A). A modern performance-line FPGA would probably hit a couple hundred MHz easily.
