One summer my commute by bicycle involved a route that regularly had a certain degree of bicycle congestion, enough that at one choke point in particular we were completely saturating the road (not just the bike lane, but the car lane heading in the right direction too).

In that portion my speed was probably halved, congestion definitely does have an effect at high enough volumes. If more roads in the area had bike lanes this wouldn't have happened and my commute would have been faster (and more pleasant, riding in bicycle commuter traffic is somewhat stressful since everyone wants to go at different speeds). On the other hand if the same number of people were in cars in the same amount of space traffic would definitely have been much slower.