The problem isn't that these weapons kill too fast, it's that they're too easy to kill with. The minimum and average kill times are too close, creating a much smaller skill gap.
With the Pistol you could get caught off guard and if your attacker was a worse shot than you you had a pretty decent chance of out shooting him despite the fact that he had the jump on you. With the minimum and average kill times being as close as they are, this translates in to mutuals more often than not. I've gotten more mutual BR/DMR kills in 4 than I have in any other game by a long shot.
Then you've also got sprint not being as punishable as it should be. Sprint is punishable in Battlefield and Call of Duty because of how incredibly fast the kill times are - one bad move and you're dead before you can raise your weapon to return fire. In Halo the kill times are long enough that it's much easier to just sprint away from battle. The issue is that increasing kill times with such a smaller skill gap and with so many power weapons available everywhere all the time is that whoever shoots first wins, and that's not Halo. The proper fix would be to nerf or remove sprint.
But it's sort of like 343 initially realized that sprint needed to be nerfed, buffed every weapon instead of nerfing the DMR or Sprint, and are now performing a 180 by releasing that new perk with the Championship Bundle that makes it so that you're the hulk and bullets do nothing to your sprint.