Async Fast Reads/sec is the frequency of reads from the file system cache that bypass the installed file system and retrieve ...

Async Fast Reads/sec is the frequency of reads from the file system cache that bypass the installed file system and retrieve the data directly from the cache.  Normally, file I/O requests will invoke the appropriate file system to retrieve data from a file, but this path permits data to be retrieved from the cache directly (without file system involvement) if the data is in the cache.  Even if the data is not in the cache, one invocation of the file system is avoided.  If the data is not in the cache, the request (application program call) will not wait until the data has been retrieved from disk, but will get control immediately.