I remember many occasions when I finish a task earlier than expected, and then always struggle whether I should stretch it and pretend it took me longer, so I can still do it faster than expected, but not disproportionally so.
The somewhat flawed logic is that if I do things too fast, I'll just get more work assigned, and expectations will rise, which will make it progressively harder for me to achieve.
I'm not entirely sure where the flaw lies exactly, but the most impressive people I worked with always seemed to be those who finished things quickly, and somehow it didn't bite them back. Others would just learn to trust their assessments, or if they are stuck would know it's not for lack of focus or trying, but that it's a genuinely tough problem.