Hello, good time As a conclusion of my experience from participation in recent codeforces rounds :

Generally Codeforces rounds in different divisions must have a standard in problems difficulty in each division, which, of course, is almost the case, but what happened if problemset difficulty be very different from usual(standard)?

If a problem be easier than you think (easier than usual and standard) if you solve the problem you have doubt in your solution and you must spend some time to review problem and solution to sure that you understood problem statements correctly and your solution is correct. this scenario have bad effect but less than second scenario Example: https://codeforces.com/contest/1372/problem/A

If a problem be harder than you think (harder than usual and standard) you spend much time in it and you expect to find a simple solution for problem base on your expectation level of the problem but your try will be unsuccessful. Example: https://codeforces.com/contest/1379/problem/A In this round me and many of participants can't solve any problem, because of that point and spent most of round time to think about problem A, Whereas if this problem is problem C(or B) of a div 2 round certainly many of us can solve that, because of conformity of the problem with participant expectation.

Problem setters, coordinators and testers please pay attention to this point, In order to make rounds better, and make better experience for people who participate in your rounds.