Hey there,
Well mainly single core clockspeed helps push GPU's to their max output (FPS). Typically Intel wins at 1080p in terms of CPU's But once the res goes up, that lead in FPS decreases. Any Intel above an i5 8600/9600k or AMD Ryzen 5 3600 or above would both drive any single high end GPU just fine. Cores do affect performance sometimes too. At least in games like BF V that can use more cores/threads. Sometimes 6 core CPU's like the I5 8600k can max out and this can cause a bit of stuttering in games. Not many though. More cores, means less maxing of the CPU resources, and can give more smoothness in games.
For most decent gaming builds now CPU's with a min of 6 cores/12 threads are preferred for more performance and reducing the chance of bottlenecks. So I7 8700/k, 9700k, 10700k are all good choices. As are R5 3600/x, R7 3700x, etc.