What is com.apple.medialibraryservice.xpc?
The com.apple.medialibraryservice.xpc is a process used for a number of Apple-specific applications and services, including: indexing photos through iTunes (when syncing), and importing and exporting images in iPhoto. 

Some users have reported that this happens when importing slo-mo videos into iPhoto from the iPhone 5s. This could be happening to you when syncing your iOS device--you can disable automatic syncing for iTunes as a temporary fix to see if helps your situation.