The slowest you can go on a highway is around 10 to 15 miles under the speed limit.
So for example if the highway you're driving on has a speed limit of 65 MPH then you could drive 50 MPH to 55 MPH.
Any slower than 50 MPH on a 65 MPH highway speed limit may get you a ticket for going to slow.
If you're driving under 50 MPH on the highway and cars or other vehicles are coming up behind you then try to pull over to the shoulder and let them pass you and then get back on the highway.
But always try to stay at least 50 MPH to 55 MPH as your speed to keep you and other safe on the road.
Driving too slow on the highway can be dangerous just like driving too fast on the highway can be dangerous.