Yes on a highway you can drive 10 miles under the speed limit.
There's a highway I drive sometimes and the speed limit is posted as 65 MPH but I don't like to drive that fast and drive only 55 MPH at most.
If someone wants to pass me I will drive over onto the shoulder until they are ahead of me and then I pull right back onto the highway and continue at 55 MPH.
If you're on an interstate and drive 10 miles under the speed limit then it might be unsafe but usually a vehicle or vehicles can just get into the other lane and get around you.
I usually drive around 60 MPH on an interstate if I'm in no hurry to get somewhere as I find going 70 MPH is too fast and too dangerous really because it takes much longer to stop in the event of someone who stops ahead of you.