Last night I ran SongKong overnight on a large collection of songs on my MacbookPro and I was surprised to find it hadn't finished, in fact it had got less through less than 10% of the songs !
Investigation of the logs files showed that SongKong had stopped doing anything 15 minutes after leaving it, and only restarted when i touch the keyboard in the morning.
Now in Preferences:Energy Saver I have the Mac configured to power off the screen after 15 minutes yet this shouldn't preventing processing, and it certainly used to work.
Turns out the problem is a new feature in OSX Mavericks called App Nap. Intending to save power it can slow down or halt applications it doesnt think are being actively used, and this seems to be the case once the Mac has gone into screen saver mode.
Thankfully it is easy enough to disable AppNap for any application.
Simply use the Get Info window for a particular application. Locate the application for which you’d like to prevent the App Nap feature
from monitoring, right-click (Command-click) on its icon, and select
“Get Info.” You can also highlight the app in Finder and press Command-I.
The Get Info window will appear and you’ll notice a new option in the
General section: “Prevent App Nap.” Check this box and your selected app
will run at full power in the background.
Unfortunately this setting doesn't seem to be preserved when you reboot your Mac.
In the next version of SongKong and Jaikoz I expect to have a permanent solution.