Sorry for not writing back but I was not at home in the last 12 hours...
It was all about the CPU temperature
After talking to some more persons about my problem I got the hint to check the CPU temperature with this tool:http://www.alcpu.com/CoreTemp/
There I saw that I had very high temperatures of 80-90 Degrees, which was obviously too much. The reaction of the CPU is to "clock down"... that's why I got a WEI of 2.9 for my i7.
So I took the CPU out and put new thermal paste on it... BINGO
Now my temperatures (of all 4 cores) are about 40 Degrees in idle and about 52 Degrees when in use... Fantastic! And the WEI? 7.6! Awesome!
Thank you all for help... I would say: PROBLEM SOLVED