After upgrading to Mountain Lion I have noticed that when I open directories there is a delay of about a second before I can see the contents of any directory. It also takes about 10 seconds to list all of the types of files in a directory, along with their sizes, when I view it.
I have run Disk Utility and it has found no errors on both the actual drive and the 'Macintosh HD' partition on it. NovaBench reports that the drive write speed is 65MB/s.
Is 65MB/s a good speed for the 320GB HDD in this Mac Mini?
In a word, yes. That is standard for a 7200RPM drive