Hi Rob,
With Mavericks, the drives are also put to sleep if they are not used for a while (even if they are ssd!), and that causes quite a long pause sometimes. A number of seconds, and with all applications that acces the file system (e.g., when you try to open a new file or folder). What you describe could be explained by this too (every time when PM gets the focus, it rescans the folders in the contact sheet for changes), but then you should see this slugginess with all applications, not just with PM.