Edit: Bungie has confirmed via Twitter that they are aware of the issue and are working to resolve it. Since the new patch, anytime multiple people join a party using party chat the framerate dips way down. My guess is this is related to the new chat feature. Leaving the party brings the game back to normal and game chat is a temporary workaround but not a solution as my group is always switching out characters and being able to communicate during that time makes everything much easier. Please prioritize this over any other unnecessary changes you had planned like arbitrarily changing the icons for elemental damage.