In most games, you'll never see "100% CPU Usage" when you hit bottlenecks on the individual threads. Games aren't programmed to utilize an entire processor, usually. You can hit a CPU bottleneck @ 50% load (2 threads on a quad-core; 4 threads on an 8-core, etc).
I'm almost certain that your CPU is bottlenecking you.
edit: Also, after looking up benchmarks for your CPU, I see most people pair that particular processor with things like a 1050ti, a 970, and (At most) a 1060. It probably can't keep up with a 1080ti.