Letting them pass. If it’s a 4 lane highway get in the right lane and drive a speed different from the car next to you.

So, slow down? If it’s a multilane road they’re going to overtake. And I think it’s disingenuous to assume they’re choosing to go under the speed limit in the fast/over taking lane.

Based on context its far more likely this is happening on a single lane road, which, generally speaking, you’re more likely to drive near the speed limit on anyway. And I’d rather someone tailgate me at a low speed than a high speed, if they’re going to tailgate either way.