Motorists are used to cyclists being slow because of this overly cautious approach that novice cyclists take to riding bikes on roadways. Another way to help retrain motorists' expectations is to do better to keep up with traffic. If you are causing traffic blockages while on a bike, then you are not riding fast enough. There are multiple ways to address this, the simplest (though not easiest) being improving your cardio and strength. A more long-term, better solution would be to redesign infrastructure either to keep cyclists separate from motor traffic or to make motor traffic slow enough that bikes don't seriously impede traffic flow when they take the lane.