FPS in raids are limited by the CPU (and RAM speed too) unless you are playing on an integrated chip or ancient GPU.
A GTX 970 can handle 60 FPS easily in 1440P.
I can tell you that because i just upgraded from one.
Now i’m playing on a RTX 2070S with a Ryzen 3900x and 3600Mhz CL15 RAM (was I5-3570K and 2133Mhz RAM before).
My FPS more than doubled in raids.
Still, i have drops below 60 on worldbosses.
On mythic Azshara i could barely hold 60.
The fastest CPU for WoW (and gaming in general) would be a 9900K at 5Ghz.
But even then you will have drops below 60 in extreme cases.
I would recommend a Ryzen 3600 with at least 3200Mhz RAM.
It’s a bit more expensive than the Intel 9400F.
But faster in Singlethreading (most important for WoW) and it has 12 threads instead of only 6 (which is way more future proof).
I would not recommend a CPU with less than 8 threads for gaming nowadays.
It will only get worse when the new consoles with Ryzen are out and games will take advantage of more than 8 threads on a regular basis.