.NET limits enforces a limit of 2 concurrent web requests to a single host as suggested by the HTTP specification. So in your case it's not the web service that needs more time to execute, but your application delaying the requests to stay inside this constraint.
You can raise the limit for the web service by adding this key to your config file:
<system.net>
<connectionManagement>
<!-- specific servers... -->
<add address="http://example.org" maxconnection="20" />
<!-- ...or any server -->
<add address="*" maxconnection="8" />
</connectionManagement>
</system.net>
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