The idea of cruising 15km/h over the limit is absolutely crazy to me. That will get you 3 points and a minimum $500 fine here. We have "average speed zones" too!
Where I live travelling at that speed will get you passed by every cop and state trooper driving on the same road. A lot comes down to local norms and enforcement.
There are different norms in the U.S. - where I am- generally 5 mph over the posted speed on side roads, and ~9 over on interstates / highways.
You are very unlikely to get stopped for either of those.
Another commented using an example of 8 and 9, but here it’s “9 you’re fine, 10 you’re mine”.