Some of Apple's biggest perf wins in the last several years involved removing dispatch asynchrony from code paths that really don't need to be asynchronous.

Obviously if you're writing an event-loop web server, then your file IO needs to be non-blocking or you're hosed. On the other hand, if you're reading a small configuration file from disk after your app launches, the most responsive option may well be to just read the bytes on the current thread and continue.

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