About 12 times faster than the original IBM-PC ?
The 8088 cpu used in the original PC had a CPU that took 3 cycles of the 4.77MHz clock to do something like a register to register ADD instruction that the Arduino does in 1 cycle of its 16MHz clock. Of course, the 8088 had a lot more memory, and a more complex instuctions set, and 16bit adds were the same speed, but the 12 times number should be about right.
The "4MHz Z80" used in CPM machines prior to the IBM PC took 4 clock cycle for similar operations, so it was about 16x slower than an Arduino.
Zippier than you'd think, eh?