Depending on how much muscle mass you gain in your chest, would determine whether it had an affect on your cycling. Obviously, muscle weighs more than fat, therefore increasing your overall weight. When you weigh more and have the same output (in your legs), then you will go slower. Also, if you add a considerable amount of chest mass, this would increase your overall chest size, along with shoulder and arm size(same muscles worked during chest workout), therefore increasing your overall thickness, resulting in you being more susceptible to wind, decreasing your speed. It really depends on how serious you are about cycling. I can't think of one guy on the tour that has an overly-muscular chest.