I partook on an EM event at Legends this weekend using the default UI and didn't have any problems opening bodies.
But I noticed unusual lag when dying when the screen was crowded and at at least 5 different times, when there was heavy skirmish nearby, I completely froze in place, unable to react. The EC didn't die, but it took a good minute or two at each time for the screen to update and the EC respond to my commands again.
This last part was likely also influenced by the 250 MS average ping I have to Legends, but I never experienced such freezing in normal game play.
With the default UI and three clients running, Corgul's AOE attack slows the game significantly on my end, but it quickly recovers after the SFX of the attack dissipates. With two clients running, the slowdown that happens is almost the same as with 3.
Edit: To clarify, I was running a single EC instance during the EM event this Saturday.