Thought about the proper play speed

How do you define slow play? What makes somebody a slow player?

People will invariably play at different paces. People who play at a slower pace, will no doubt, not think they are slow, just playing at their own pace. But if it disrupt other players or overall schedule of whole people’s play?

But 5 or 6 hours for 18 holes are ridiculous.

It’s hard to say the time limit of the slow speed, any proper way to define it?