In general, congestion is nothing to do with cycling, but is a direct function of the volume of motor traffic on the roads. Were this not the case, then we would only see congestion on urban roads, and never on motorways (where nobody cycles).
Congestion on roads is due to motor traffic, and in particular, queues at junctions. While driving behind someone cycling is going to slow you down if you’re in a car or on a bus, it is unlikely this is going to add any overall delay to your journey. You will arrive at the tail end of that queue in exactly the same position – driving behind someone cycling has not cost you any extra time.
Reallocating space on roads for cycling will actually make roads more efficient at moving people – a typical motor vehicle lane can carry around 2,000 people per hour, but the same space allocated for cycling infrastructure could carry around 10,000 people per hour.