We were grinding Gyfins today.
The FPS dropped (depnding on the system) by amost 60-80 FPS everytime a mass burst on damage was done on about 10 MOBS.
The CPU load went down from 3-4 CORES down to 1 CORE, clearly stating the client is either waiting or busy with some calculations on a single core.
When you disband the group and just do damage to the mobs the FPS drop was nearly 0 FPS, indicating that this is an issue with the client.