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 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.

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