Experienced this tonight in Halitai HM. I didn't notice any lag but it was pretty rough keeping hate off an ok geared dragoon and bard. I'm using melded I70 accessories too as PLD with around 420 STR. Flash was barely making a dent on healer aggro. Each person was taking a different mob which is fine as I would drop whole RoH combos. When rotating through mobs I could barely keep hate.
It felt like I wasn't using Shield Oath. I've started fights without it before and it seemed very similar. Especially with how useless Flash seemed. I cycled it on and off a few times but it didn't seem to help.