Have you tried giving people useful feedback? It often ends in hostility, no matter how nicely you put it.

Ultimately, it is not my responsibility to ensure people are performing to adequate levels of DPS. I'm happy to help if you want it, but not at the expense of the run I'm currently trying to do. Work on your DPS on your own time, and then join runs requiring serious performance! Guides and training dummies (and parsers) exist for this purpose!