The stated propagation delay for TTL chips was usually about 10 or 20 nanoseconds (sometimes larger for more complex logical operations, and frequently smaller

The stated propagation delay for TTL chips was usually about 10 or 20 nanoseconds (sometimes larger for more complex logical operations, and frequently smaller in subsequent logic generations, like 5 nanoseconds or so). I just checked this in a late 1970s databook. So, 20 MHz seems pretty credible for a region where you would see problems due to propagation delay—maybe even somewhere below that with the original 7400 series. That's also a reason that I hadn't thought about so much why putting the whole CPU on a single die would look more and more practical over time: propagation delay in wires!