Your task is using all of a single core, not 12.5% of one. 12.5% is 1/8 - you're maxing out one of the eight hardware threads your hyperthreaded quad-core processor has. If you disable hyperthreading, you'll have only four hardware threads, so your tools will report 25% usage - but your task won't complete any more quickly.
If you want to use more of your processor, try to parallelize the work.