Traffic flows better at lower speeds. This may be contrary to what people might think but the reason is very simple : the lower the traffic speed, the less space is required ahead of vehicles for stopping distances. This makes a more efficient use of the road space and this is why we have (lower) variable speed limits on motorways.
Lower speeds also means that the roads can accommodate more vehicles passing through an area at any given time and so can therefore ease congestion. If we want heavy traffic to flow, it needs to move slowly.
Lower speeds also mean that it is easier and safer for vehicles to emerge from side roads and join the traffic flow. Not only are reduced traffic speeds safer they can also reduce driver frustration and tension.