You should definitely try to keep that in mind. I had that issue as well, trying to save everyone. In dungeons I only care about winning. If I feel that anyone is becoming a burden I let them die. Tanks included, especially ones that cannot hold hate. I've side stepped and healed around people so they would die faster because they were causing too much chaos on my field.
Odd thing, but try to reload your ui before you start the fight. I've noticed, not all the times, but a good number of times the UI is causing graphic lag. Refreshing it helps me get rid of that. I have noticed server lag too so it's very possible it could be that as well.