they did a major overhaul to the particle effects in WOD and legion. it made the game unplayable for a lot of people.
at first these new effects were so poorly optimized that the requirement to run the game went up 10 fold in certain area’s
a good example was that pig boss in Razorfen Kraul who would do a stampede of smaller pigs that would fill up the ground with dense dust effects. it would cause my GPU fans to run at 100% power.
or the boss in Utgarde Pinnacle who’s dragon fills the walkway with blue smoke.
they seems to have toned down these effects somewhat by now. but they are still poorly optimized…
i would not be surprised if people struggle to keep good fps in classic 40 man raids if they use this new particle system.