You were likely cpu limited in your game.
It was likely mostly single threaded.
Be careful how you interpret task manager cpu utilization.
Windows will spread the activity of a single thread over all available threads.
So, if you had a game that was single threaded and cpu bound, it would show up on a quad core processor as 25%
utilization across all 4 threads.
leading you to think your bottleneck was elsewhere.
Not much you can do except change out the processor.
You could play with quality settings and resolution.
Possibly there are game settings that will help.
To get a check on the cpu limitation theory, run this test:
In windows power management REDUCE your maximum cpu from 100% to perhaps 80%
If you see a corresponding drop in performance, core speed is the issue.