The implementation of Dns.GetHostAddressesAsync is just a wrapper around the blocking implementation and is queued on the ThreadPool.

https://github.com/dotnet/corefx/blob/1baa9ad06a466a5d6399e1e0a7a7a02564ab51b0/src/System.Net.NameResolution/src/System/Net/DNS.cs#L274

This can lead to a ThreadPool starvation when there are many concurrent calls of GetHostAddressAsync in an environment where the DNS is slow.

The code in .Net 4.6 is slightly different but suffers from the same issue:
http://referencesource.microsoft.com/#System/net/System/Net/DNS.cs,744

Windows and Linux both provides real asynchronous calls to resolve a DNS. The asynchronous implementation could use these.