Evaluating a scenario, we've created a .NET user-defined function which
does nothing more than return a bigint from a previously initialized
static field of a class (this value is initialized on system startup,
and holds a unique runtime value, for reasons unnecessary to go into
here).

Running on a beefy dual-Xeon 2.8Ghz 4GB machine, with nothing else
running, this yields a call rate of just 40,000 calls per second. Of
course a .NET internal function call happens more at a rate of about
100,000,000 per second on similar hardware.

Is SQL Server building up/tearing down the class? Is this that much
overhead? Is there anything we can do to improve the integrated .NET
performance?

SQLXML 4.0 "timestamp" updategram question
Identity keys in a One-to-Zero-or-Many relationship
Insert in every ID
Function results
Incorrect information from a "View"