I'm pretty sure the CPU is still number-crunching the spell-effects of the player, the boss, and some low-level version of the surrounding players doing the same thing. GPU will render the image, but whatever low-seed, low level of detail, and position, that's still a cpu workload. Back in garrosh 25man, you'd get your own high level effects +24 other players on high level detail + the boss + the trash + a big ball of death with fire = 120 fps average drop to 32 fps on the most enthusiast setup and max stats. After Garrosh aka Warlords of Fails, Blizzard heard the cries of all the low-level computer setup-players, and made some changes. They made it where your fps wouldn't take a massive hit while 24 to 39 other players casting their CDs around you. The level of detail of other players' spell-effects in raids that wouldn't drop your FPS. I'm pretty sure D3D12 made life a little better for the those low-level computer setup-players to see better FPS with low-end GPUs while casting "pew-pews" with 39 other noobs. When this majority of the WoW subscription base is happy, they play WoW more, and pay their subscription for a longer duration of time because at the core, WoW is a business. This also means that if WoW is doing better, and they are drawing a bigger crowd, the majority of the base of players have low-end computer, and blizzards need to make changes to accommodate them in exchange for more $$$. This is probably why they implemented D3D12. If it means more players with crap-tastic computers will play and pay, Blizzard-Activation will implement. Investments FTW.
