Now before people freak out and start yelling noob let me explain:
Imo harder dungeon content should come from more dmg, more health or the affixes like there is now. Where does a time limit fit in there? It doesn't make it harder, just more frustrating.
All it promotes is rushed content. One of the problems people had with challenge modes was it felt like a rush fest. How is mythic+ different?
I've been tanking several +5/6 mythics now and all the time i was thinking: must pull faster, must pull faster, must make the time limit! The fact you always have a guy yelling gogogo! doesn't help either (we always made the time limit but you always have 'that guy')
Imagine if mythic raids had a time limit. The outcry would be immense but for dungeons its ok to have a rushfest?
The time limit promotes (at a high mythic+ level) certain class combos purely because of the time limit.
Hard dungeons are perfectly reasonable with just using dmg/mechanics.
TLDR: harder content should come from more mechanics and dmg/health. Not a rushfest a time limit gives