Even for a triathlon, the speedo, while it will facilitate dissipation of heat from the equipment laden first two legs of swimming and cycling, would ideally be modulated with the addition of extra clothing later on during the run. That means were the run portion more of a sprint, out of proportion to still longer running and cycling legs, then indeed a speedo would be used the whole way, it would interfere with speed to put on Extra gear in a shorter race. However, more clothes are useful for providing stability. Basically you are a ship going at high speed, cutting through the elements, even on a boring course or a track. You’ll have some sails, etc. and you don’t want to freeze. However if you are practicing in the sauna mentality then you may have to do it.