The Nexys-3 board on which it runs is clocked at 100MHz, the 16-bit CPU executes a single instruction per clock cycle. I have not tried it on any new FPGAs, just on the board I have, it might run faster on a newer FPGA. It can also respond to an interrupt within 50ns (but more like 90ns with the overheads of setup in the interrupt handler).
It's not meant for high performance - but it is tiny! The program is limited to 16KiB in size, of which 6708 bytes are used by the eForth interpreter that is running on it. It's a very niche thing.
