Usually, I only get short with them if they're either significantly ( i.e. 15% or more under the speed limit) or if they're up and down ( drifting between 90 and 110 for instance)
Of note is that here anyway, it's illegal to drive on the shoulder, and you can be busted for it. So,... if friendly cager moves over for you, and you're passing, they can be busted. It's a bit of a deterrent here. Many of us do it still, but carefully, when it's clear of the damn sheriffs that are just glorified tax collectors.