Hello jeroompje
Based on your description, I would like to advise you to use the SSD on the SATA ports 0 or 1. If The SSD is used in a different SATA port is possible that TRIM may not work as expected. You can check the following link for more information:
http://www.intel.com/support/ssdc/hpssd/sb/CS-031846.htm?wapkw=trim
Also, I recommend you checking that TRIM is enabled. You can check the following link to get the info in how to enable the feature:
http://www.intel.com/support/ssdc/hpssd/sb/CS-032318.htm?wapkw=trim
It is also important to mention that the Intel® Solid-State Drive Toolbox software is important to be installed in the system.