Ghost (setting)

[PlainDealingVillain]

Waking a typical ghost is doable if you know where they are and have something to get their attention with, the difficulty is interacting with them because the timescales don't have to get that far out of sync before it's very inconvenient. And paper does not generally become ghostly, so writing messages takes some doing. (Blackboards are currently the most common usable ghostly writing surface; chalk is a little hard to come by, but it works OK, and physical chalk dust on ghostly blackboards works.) Once a ghost has gone full fugue, it's such a slow process to communicate that most people can't maintain the effort without draining themselves and slowing down.

There are about 500 million active ghosts worldwide. (This is about as many people as die in a decade.)

There is probably not a limit to how fast you can go, other than the psychological one that you have to maintain very strong emotions. Which is hard.