Why did this solution TLE but it passed when I converted the lambda functions to normal ones? Am I doing something wrong here or can lambda functions have this big an overhead?

Why is the cumulative time for all submissions used for ranking in educational rounds? Doesn't it make more sense to take the time of the last correct submission for ranking? The order of the solved problems should not be relevant in my opinion, if X solved A-D in lesser time than Y then X should be ranked better than Y irrespective of whether X solved the tougher or easier problems first. Are there cases where the cumulative score makes more sense?

Edit: On more thinking, I guess it discourages people from solving and sitting on solutions till they have a certain number of solves.