I don’t have the game go test, although I’ve been meaning to give it a try…
It sounds to me like it’s limited by CPU single-thread performance, which isn’t suprising at those frame rates. Looking at overall CPU usage doesn’t tell the whole story. It’s likely that some threads on your CPU are active 100% of the time.
You may have some headroom on GPU performance, so you could try increasing quality in a way that isn’t likely to impact CPU performance. I’m not sure what options the game has, but some good candidates would be: rendering resolution, texture resolution, anisotropic filtering. If you can do that, and it increases GPU use without dropping overall performance, it more or less confirms the CPU bottleneck.