Green speed is determined by one simple thing: Friction. The less friction, the farther the ball will roll. Things that increase friction on greens are higher cut turf, over fertilized turf that creates a succulent turf stand or simply rain fall. Green speed is technically not a speed, as in miles per hour or other unit of distance traveled over time. It is measured in terms of feet the golf ball rolls from the base of one of these 2 devices:
The pelzmeter (left) and the stimpmeter (right).
Dave Pelz developed this device to give more accurate green speed readings. This device operates off a key law in physics: an object losses speed rolling up hill faster than an object gains speed rolling down hill. Think about this. In the case of the stimpmeter, it is a true average meaning you take the sum total of the ball roll one direction plus the other direction and divide by 2. Theoretically, a ball could roll 20 ft downhill off the stimpmeter and then 2 feet uphill. According to this, the green would be rolling 11ft. However, that is not the case. Taking the Pelzmeter formula into account, the green is actually rolling 3'8". Far from 11'! The point is, a superintendent (if not using level ground) can think the greens are rolling much faster than they actually are.
To determine green speed, I will find a relatively flat area on a putting green. I will roll 3 balls one direction and note the average distance they rolled. Now, I will go the other direction and do the same thing, noting the average of those 3 balls. This morning, on the putting green, I got 14' one direction and 9' the opposite direction. 9 + 14 = 23. Therefore, according to the stimpmeter, the greens are rolling 11.5' because 23/2=11.5. On the pelzmeter however, I did get the same distances. But, using the built in computer and accounting for the laws of physics, the greens are rolling 10'11". Simply put, the pelzmeter is more accurate on greens with a lot of slope. The stimpmeter is only accurate on a completely flat portion of the green.
The USGA Stimpmeter was developed for golf course superintendents only. It was a device used to compare the speeds between greens around the golf course. However, it eventually became a tool to compare courses which is not fair. The more slope a course is designed with is different compared to one with much flatter greens. Think about it like roads. You may want to drive 80mph on I85 through Atlanta. But, would you do the same on these mountain roads?