It's a combination of practice and raid set-ups. A lot of these speed-run groups go for a NIN, DRG, BRD, BLM set-up for the insane burst and AOE damage. Only use their Stances to mitigate in-coming damage/hold aggro, so they can just pew pew 90% of the time.

Even myself, I barely pull 570DPS on T10 and I'm just as geared as everyone else now.