Imagine two kinds of days you’ve probably had. On the one you have a short to do list, on the other you are entirely swamped. On both days you spend a full working day finishing your tasks, even though they both have the same number of hours in them.
This phenomenon is referred to as Parkinson’s law (nothing to do with the disease):
Work expands so as to fill the time available for its completion
In other words, make sure you budget the minimum realistic time for a task. You will find that suddenly you can do far more in a day.
What is important here is that you allot a minimal amount of time to a task, but keep it within reason. Know thyself.