As a US citizen I will tell you this is false. We have basically no national traffic laws.
Your local state, city or other local government may have passed a law like this.
For example, in my area, it was made illegal to stay in the left lane moving more slowly than those behind you -- you must yield to the right if they want to go more quickly.
But some 99% of Americans do not live under this local law I'm discussing.
There are some highways that have a "minimum speed limit" sign posted. Not often, but they do exist, and you can get a ticket on those highways.
Also, a Google Self-driving car got pulled over the other day for simply going the speed limit, it did not get a ticket but I thought I would mention it.
You're right, it's not national. But as far as I'm aware, all states have general rules against impeding traffic. Many don't have explicit laws against moving too slow in the left lane, but the general law can and will be used against you for egregious violations.
As a US citizen I will tell you this is false. We have basically no national traffic laws.
Your local state, city or other local government may have passed a law like this.
For example, in my area, it was made illegal to stay in the left lane moving more slowly than those behind you -- you must yield to the right if they want to go more quickly.
But some 99% of Americans do not live under this local law I'm discussing.