Implementing Logic Functions Using Only NAND or NOR Gates …
If we were to say that NOT, NAND, and NOR gates each equate to one level of delay, while AND and OR gates equate to two levels of delay, then the worse-case input-to-output paths in our original NOT, AND, and OR implementation would equate to 1 + 2 + 2 = 5 delays. By comparison our spiffy NAND-only implementation equates to 1 + 1 + 1 = 3 delays.