I worked on some 6502 based stuff in the early 80s, thanks to a college internship. As I learned from my mentor, the original 6502 was NMOS and had some dynamic registers, i.e., their contents needed to be refreshed, thus limiting how slow the machine could run. But the 65C02 was CMOS with all static registers and could be slowed down to zero through appropriate control of the clock.

The thing we were building for my internship used a very low clock speed to conserve battery power, since CMOS draws current only during transitions.