And yet if traffic is traveling at the speed limit and you’re going 20 MPH slower you’re a hazard.

Most cops will likely even pull you over for that.

I’ve seen more than one person try to merge onto a highway at 45 MPH and every time they very nearly caused an accident.

I often ride my ebike at 28 MPH (max legal ebike speed in US) on a street with a 25 MPH speed limit and I'll regularly get cars tailgating me honking and crossing the double line to pass.

American here, in practice no. They are generally only the upper bound for dense neighborhood streets, throughout most other roadways you will be considered a poor driver if you go lower than the speed limit.

I know two people who have been pulled over in the US for driving too slow.

If you drive significantly slower than the flow of traffic it can create a hazard for other drivers.

Generally speaking, much of driving is predicting what other drivers will do. If you behave significantly differently than other cars you do raise the chances of an accident.

Imagine how many diesel exhaust clouds this clown car going 25mph is going to be served at green lights in the bible belt of the US