As I understand it, the Windows caching layer doesn't actually support asynchronous IO. Any asynchronous cached file IO runs on a small, fixed-size system-wide

As I understand it, the Windows caching layer doesn't actually support asynchronous IO. Any asynchronous cached file IO runs on a small, fixed-size system-wide thread pool using blocking IO, and once that runs out all such operations block until completion. That's why Windows software generally uses its own thread pools for file IO.