It takes just one member getting hate with an AoE attack and suddenly they're eating not one attack but ten attacks and there's nothing a healer can do about it anymore (but still get blamed ba-dum-tsh!).
Speedruns are scourge on this game, you're causing an incredible amount of stress on the tank to keep hate and the healer to keep people alive, and what does it realistically gain you? A few minutes saved running in exchange for a tank who is instantly done for the day because he cannot handle it anymore? Sounds great, doesnt it. Before you say otherwise, lets face it, most common speedrun groups are not going to shave off more than a few minutes.
Heck, Speedruns have caused me to stop playing on my Paladin and have made me do dungeons on a DPS class instead. And i'm surely not the only one. So here's one for you, want to solve the problem with there being so few tanks looking to do dungeons with you? Try being nice to them and stop forcing them to speedrun things.