Rounds of tests by TechEmpower from April and May 2019 demonstrate positive results, performance-wise, of BlackSheep framework.
High performance, relatively speaking for a Python web framework (still much slower than some modern frameworks built with a statically typed language, like ASP.NET Core using C#) is mostly explained by the use of uvloop (libuv), httptools (http-parser), and asyncio. The performance is similar to the one of ASGI frameworks based on Uvicorn (like Starlette and FastAPI), using the same library for the event loop (uvloop).
The main reason why BlackSheep doesn't implement ASGI specification today, is because it uses custom extension types to handle entities such as HTTP requests, responses, headers, instead of generic types
list. Since these built-in types are extremely efficient, it makes sense that the performance offered by these two strategies is similar.