For this problem : https://codeforces.net/contest/1805/problem/C, I submitted this solution: https://codeforces.net/contest/1805/submission/215278227.
The runtime is 78 ms.
I then submitted the exact same solution with a simple comment at the top : https://codeforces.net/contest/1805/submission/215278404.
The runtime is 93 ms.
Although meaningless here, what accounts for the difference in runtime? I would understand if the difference was 1 or 2 ms off, but 15 ms off seems like a lot for two exact same solutions.
15ms is a very short amount of time.
In addition, Codeforces likes to round to near-multiples of 15ms. If you look at submissions to the problem, you can see that out of thousands of accepted solutions, none have run-time between 78 and 92 ms due to the Codeforces rounding.