The social stigma against speedrunners comes from the commonality with which one will appear in a Duty Finder roulette and, one of the following tends to occur based on class:

-If a tank, they'll immediately rush ahead to take no less than four sets of trash mobs without even inspecting their healer or giving a warning, fail to rotate cooldowns, and complain when they die.
-If a healer, give the tank and DPS grief for not going fast enough, while they themselves do nothing to contribute to dungeon speed because they'd rather afk and watch youtube than cleric stance and pop a couple of dots or Holy to do their part.
-If a DPS, passive-aggressively remark that the dungeon is taking too long while clocking a pathetic 180 DPS or something equally bad and not being able to find their limit break key on boss encounters.

Speedrun groups are a blast and I enjoy the activity of maximizing my output on content that was outdated the second it was released.