The test was to run "SELECT 1" using the low-level ADO.NET database query API in a tight loop. This is the relevant metric, as it represents the performance ceiling. It doesn't matter how fast the packets can get on the wire if the application can't utilise this because of some other bottleneck.

Of course, the underlying TCP latency is significantly lower. Using Microsoft's "Latte.exe" testing tool, I saw ~50 μs in Azure with "Accelerated Networking" enabled. As far as I know, they use Mellanox adapters.

Something I found curious is that no matter what I did, the local latency wouldn't go below about 125 μs. Neither shared memory nor named pipes had any benefit. This is on a 4 GHz computer, so in practice this is the "ultimate latency limit" for SQL Server, unless Intel and AMD start up the megahertz war again...

It would be an interesting exercise comparing the various database engines to see what their latency overheads are, and what their response time is to trivial queries such as selecting a single row given a key.

Unfortunately, due to the DeWitt clauses in EULAs, this would be risky to publish...