Well if you want to take it slow go with friends or make a PuG with exactly THAT in the description.
If i run a dungeon for the 20000 time i don't want to sit around and wait for ages because one person just "wants to take it slow".
The difficulty of Mythic plus comes from the timer. Getting through is not hard with infinite time. I (for example) ONLY do mythic+ because i want to push my Raider.io and the challenge is fun. If i run heroic i ONLY do that to get the inital gear for harder stuff. An in LFD in will never see the people i play with again so just power through it and be done with it.
I don't go into mythic+ and say it is just a 5 man. I go in and say i want to finish it the best way possible so i can get to harder stuff i can tackle