For example, if you have 3 tasks to do every period of time, but you manage to do only 2 in that time period. You're falling behind, but if you want to measure how far behind you are, what word could you use ? Is there a single word for that ?
If you wrote on a spreadsheet how many tasks you're behind, how would you name the file ?
"Lateness" and "delay" don't seem mean that, and I only find verbs in dictionaries.