Today after the round #707 I managed to get Time Limit Exceeded on problem B. Surprisingly, when I resubmitted after system tests, and got AC fitting tightly into the TL.

So I resubmitted $$$5$$$ more times. Each one got Accepted, too.

It's not a first time this happens. During the round #683, when I was among writers team, in one of the problems someone managed to get TLE on system tests even though we had pretests=tests.

Did you also experience something similar? Any idea what causes the judge to become slower during the contest? And, most importantly, should it be considered a bug or a feature?