Speed on a bike is irrelevant unless you are a pro who makes a living out of it and gets paid to ride....The most important thing is to focus on what you enjoy doing. If multi-sport training makes your life more interesting and enjoyable then continue doing it...For me personally I find that kettlebell training has most carry over to cycling or any other physical activities I don't have any science behind it but all I know is that if kettlebell training is done correctly, it works and helps me to perform better in other activities such as singlespeed mountain biking. Deadlifting also seems to have a beneficial effect... Doing only one activity usually ends up in overuse injuries , so a variety of different activities is best.