It's quite common during competitive programming (algorithms and datastructure). In many of the of contests your given input size constraints and time limit. Knowing that helps you narrow down which approaches or tools you shouldn't even try because they would be too slow, or when to stop because you came up with something good enough.
In few contests that don't give precise input limits, it adds extra challenge. And if your idea for the optimal efficiency is wrong, you have not only wasted time implementing wrong solution, but might also get a penalty score for each failed submission. One of the worst feelings is when you see that your submission got time limit on something like test 87, and you are unsure whether it's due to having wrong asymptotic complexity or whether you have correct algorithm but just implemented it slightly too inefficiently and it only needs a little bit of minor optimizations to pass.
In few contests that don't give precise input limits, it adds extra challenge. And if your idea for the optimal efficiency is wrong, you have not only wasted time implementing wrong solution, but might also get a penalty score for each failed submission. One of the worst feelings is when you see that your submission got time limit on something like test 87, and you are unsure whether it's due to having wrong asymptotic complexity or whether you have correct algorithm but just implemented it slightly too inefficiently and it only needs a little bit of minor optimizations to pass.