PC processor limited to 66%

My Windows computer, when I put a lot of strain on it and open task management, always shows me that the processor is 66% used and never more. This limits me a lot in using the computer, because it is much slower.

I would like the processor to be able to be used 100 per cent.

Anyway, here is the processor: Intel Core Duo E8400 3.00Ghz

I have searched the Internet many times about this problem but have found nothing. It seems as if there is a limiter on the processor that would have a power of 3.00Ghz, but only uses 1.98Ghz.

With another app (OCCT) this problem seemed to occur: the performance graph was up to 100%.