My first Apple II had 4k of memory (storing programs and data) and ran at 1MHz. Arduino runs 16 times faster and the latest versions have 32k of program memory and 2k of RAM. But comparing computers with different architectures is like comparing apples to ? (well, you see what I mean).
But if you want to express some idea of how fast Arduino can be, it can switch a pin on or off (using direct port io) in less then the time it takes a light beam to travel 20 meters.