TRS2004 and older versions of Trainz store the kuid files in human readable format but the Trainz program has to convert the data in the kuids into machine readable (binary) format each time you select a route place an item in Surveyor.  This slows things down, especially in large routes.  Starting with TRS2006, Auran changed the kuid storage system to a database format to decrease the load times when you run the game.  The database format requires kuid files to be converted to binary format for storage, a process called commiting.
 
Bob