World of Warcraft is a prime of example for overly relying on a strong single core, and even with over the top GPU power can crash and burn at times where you FPS simply tanks into the ground.
Add in over 80 people all duking it out in one spot, or an overwhelming horde of enemies and your'e going to drop FPS into the teens, all the way down from averages of the Hundreds.
FPS in top right corner under the mini map- First image is with SLI 980Ti Superclocked cards, second is with Fury X - Both with 5820K @4Ghz
This was the case for me with everything from E8400, Phenom II 940 BE, Xeon W3680, and current 5820K 4Ghz.
With GPUs ranging from 6600GT, 8800 Ultra, 4870X2, GTX 470, GTX 680, GTX 980, GTX Titan X, GTX 980Ti SLI, and Fury X.
Those nasty drops are essentially down to the game's engine being extremely old, and relying on a single CPU core mostly, with lots of current tech bolted on over the years.
I have people in my WoW guild with new systems with 6700K overclocked over 4.5Ghz and GTX 1080's all still suffering the same nasty FPS drops in similar situations as other members with i5 2500Ks at stock, or 4670Ks.