It sure sounds like you need to do a secure erase and reinstall. I've heard that you need to leave about 15 to 20 percent of the drive free but there is so much misinformation on the web I didn't know what to make of it. What does the Intel Toolbox say about the drives health?
If you end up doing a secure erase please report back on your results to this forum. I for one would like to know if this solves your issue.
I found the reason for my performance issue: the McAfee Endpoint Encryption
I deleted the partition and installed a fresh windows 7 -> 250MB/s read performance
installed the McAfee Endpoint Encryption 5.2.3 -> read performance slows down to 110 MB/s
Disk ist 50% free
I checked the McAfee knowledge database, SSD performance is a known issue with the full disk enclyption and should be solved with the newer Versions
I don't belive that my company will invest money a new version next time, so I have to live with the slower performance