If you can’t get everything accomplished by 5:00 on a regular basis, then the job you have might be a bit too big for you.
This is such a great post and so true.
Geoff Yeagley

Alternatively, you might not be carving out enough time to focus on the task at hand. If I recall, Cal Newport suggested chunking work into distraction-free amounts of time (1–3 hours?) to be productive.

So perhaps not finishing on time could be because of the constant tug of IM, email, Slack, and not yet being used to cordoning it off to certain periods of the day?