What should you do when traffic in your lane is moving much slower than the speed limit?

You should drive at a speed appropriate to the flow of traffic and conditions, not blindly at the posted maximum. When traffic is moving slower than the limit, match its pace safely rather than tailgating or weaving. At the same time, driving much slower than the surrounding traffic when conditions allow normal speed can itself be hazardous and, in some places, illegal — slow drivers should keep to the right and let others pass. The goal is smooth, predictable movement with the flow. Balancing the posted limit against the actual flow and conditions is part of safe speed management on the road.