Did you try compiling all the test to native thru AoT? There are probably some optimizations to improve the Kestrel score by altering the config, or maybe changing how the API is written:

app.MapGet("/api/test/hello", () => "Hello World!");

As a general web server, I think I'd always go for Kestrel over this, but as another poster said, if you're trying to add an endpoint to some desktop application, this would be perfect.