How can a library providing only sync functions a good thing? Lots of IO is inherently async and it makes little sense for a library developer to go out of his way to make it synchronous only for the callers to make things asynchronous again. If something is async, it's async. You deal with that.

Also, I don't see why one would even consider using a framework if squeezing out every bit of performance from your hardware is that important. I can't imagine writing a Web app without some framework in the interest of time it takes to develop. Then writing a hundred of the same stuff again? You just have to ensure you don't use a bloated framework which is akin to not choosing a bloated library. Unless you are working on a device layer, it's not our job to spread things over the hardware. Unless you have direct access to memory, you can't do this anyway with most of the languages designed to develop apps. I think the primary goal of an app developer is to implement the business logic correctly and securely in a reasonable time. I agree with the OP that libraries should be preferred but I don't see a problem especially with performance in the domains frameworks are used.