By default .NET allows only 2 connections to a given network address per AppDomain. In most cases this works fine but if your app makes a couple of dozens network calls a second then this value might be too small and it might actually cause a bottleneck that is very hard to diagnose. I decided to increase the value of this setting to the value that is recommend by Microsoft (number_of_cores x 12) and one of my services speeded up significantly. Having said that I have to stress that there is no guarantee this setting will work in your case. Remember, measure, measure and once again measure when you optimize.
And the setting is:
<system.net>
<connectionManagement>
<add address="*" maxconnection="96"/>
</connectionManagement>
</system.net>