A number from a simple test
Green speed is typically quoted using a stimpmeter reading, taken by releasing a ball down a fixed ramp on a level part of the green and measuring how far it rolls. A higher reading means less friction and a faster surface. The measure is simple, but its effects compound: on a fast green even a gentle slope becomes decisive, and a putt struck with the wrong weight can finish yards away rather than inches, turning routine putts into genuine risk.
It punishes approach play too
Because faster greens are usually kept firmer as well, an approach shot that lands even slightly hot will release further past the flag than it would on a slower, softer surface, so players must land the ball shorter and accept more roll. This means green speed is not confined to putting statistics at all; it reshapes which pins are attackable and forces approach shots to be judged by where the ball will finish after it bounces, not simply where it lands.
A deliberate difficulty setting
Major championships routinely raise green speed above a typical weekly tournament, and they do so on purpose, knowing it will produce three-putts and cautious lag putting from distance. This is a difficulty lever pulled independently of length or rough, and it explains why a course that seems generous off the tee can still play brutally hard once the ball reaches the putting surface, where the real examination has been waiting all along.