Without seeing the code it is impossible to know the reason of the slowdown. But 6-8% is really small. Do you have a machine with 16 cores? Then this number might mean, that only one core is active. But it is more likely that your RAM is exhausted and the CPU is waiting for the very slow virtual memory store on the disk. Please check this.
You can get a more specific answer, if you use the profiler to find the bottleneck of your code and post it here. This will be more successful than guessing, where the bottleneck might be.