You could look it up

dominic introduces the new read-write API for the U.S. National Archives' online catalog. Plus-one from me for providing (rate-limited) access without an API key or authentication -- quick start mode, if you will. I also like defaulting to pretty-print.

And search results return a field queryTime, which looks like it's the number of milliseconds that it took to execute the query (the interactive documentation isn't clear about this): of some value to client software, but very useful for developers maintaining and performance-testing the API itself.