I not agree.
Particularly about tactic puzzles, the time involved to solve it would be long, as in the real matches.
The time limit may be useful for chessmate puzzles, but not for tactic.
I believe that the site is devoted to learn how have the best thinking method to win in a particular position, not to win fast.
My suggestion is that, if some elapsed time must be taken into account, this is for the Elo computing result.
I apologize for my English.