Could you elaborate a little more on "wouldn't do much if the particular tasks aren't completed within scheduled period due to loss of focus."?
Do you mean that accomplishing 80% of a task isn't substantially more valuable than accomplishing 0% of a task?
(If that's true, then it might still be possible to improve the situation by:
A) choosing an appropriate time when one has the ability to accomplish all 100% of the task at once
B) analyzing the factors that prompt focus to drift, potentially distractions could be out-competed by other focus reminders as you've noted
C) putting some more thought into ways to split a larger task into pieces that feel separate )