For the OP's question - Slowing down is the only safe thing to do in that situation. Speeding up wastes a lot of fuel, takes a lot of attention from the road, and puts you in the other cars blind spot, and then the other car in your blind spot as you pass.
You do have the right of way if you're already on the highway, however a very small adjustment in speed is usually all it takes to help someone move - never more than 5 MPH in my experience.
I used to drive a semi truck governed to 60 MPH in states with 70 and 75 MPH speed limits - this technique worked very well for me. I still got flipped off a few times (Some people are total idiots that do not pay attention to anything), but for the most part it works.
Also, in the 75 MPH states I would take the backroads, such as US-30 instead of I-80 in Nebraska, which got rid of the entire problem and gave me a far more relaxing drive.
I'm not even going to mention my opinion of tailgating trucks - other than I'm so happy that some states are finally joining the TACT program
