I’m a traffic engineer and I’ve literally never heard of such a thing (except on freeways).
In Georgia it’s illegal to travel 15 or more under the posted speed limit, including on non interstate roads, unless otherwise posted.
Edit: getting downvoted for saying facts about my state. Yall are weird
In Sweden it can be illegal to drive too slowly. You’re not allow to hinder trafic and should follow the speed limit. At the same time we have tractors and vehicles that are only allowed to drive in 30km/h but they require a special sign on the back to signal to other people that it’s a slowly moving vehicle.
It’s however incredibly rare that anyone gets a ticket for driving too slow.
There isn’t a set value. You can see sources below.
Georgia: https://dds.georgia.gov/section-5-continued-keep-right-except-pass
National: https://www.nhtsa.gov/sites/nhtsa.gov/files/summary_state_speed_laws_12th_edition_811769.pdf
