If your program is slow because it's waiting for external service response, you're doing programming wrong. Your program should do other work in the meantime. I guess you're already doing that, but if so, doesn't that invalidate your reply here?
It's not always true that there is other work to do in the meantime. In fact in my experience, it seldom is. "you're doing programming wrong" is a very strong statement, and not one that I take seriously in this context.
Typically you "await" the external service response, so that it is not using a thread to do that, and "other work" in the form of starting to deal with other requests can happen in the meantime, thereby increasing service throughput.
But that won't speed up a given request - you can wait for an external service more efficiently, but you can't wait faster.
Services that do not depend on any other http services or any data store do happen, but they are rare in my experience (calculation engines, I suppose). So for almost every service, when thinking about response time, you have to, first and foremost, think about the latency of the data stores or upstream services.
If You want to be pedantic - and you most certainly do - then only .NET performance itself is relevant to performance of . NET itself, that's a truism, as defined.
But this narrow focus is not useful - if you want to do the job, then you have to think a bit more widely, and understand what the real problem is.
I am not saying that; you are oversimplifying into a straw man. great-grand-parent comment is where I literally put a non-zero number to how relevant language perf improvement was: https://news.ycombinator.com/item?id=29295950 and later on "Both can be relevant"; which all contradict your characterisation.
The only person who said above "it's not relevant" is you, and you also said "external services are irrelevant" - You're projecting the "it's irrelevant" statement onto me here.
But the design considerations of how and when to use external services very definitely are relevant to service latency, contrary to what you say, for reasons given multiple times above. Your current odd comments are not fact-based or interesting, so I don't think that you have anything more to add to this discussion at all.
What work? Mine bitcoins while you wait the result for an API call?
If you need to improve this, and that code is .NET, then the solution is a different design, also in .NET Code.