[OniBait]

I'm curious where the 40x slower figure came from. It would really depend on what you are doing inside the method.

If the method is a small amount of "synchronous" code in a tight loop, then yeah, the overhead of the async call is going to outweigh any benefits (a few orders of magnitude slower).

However, if your method is doing something like reading in a 40MB file to do some processing and calls the ReadAsync methods then the difference will be negligible and you get the benefit of better parallel throughput.