First, when running the Windows Experience test, you need to close all other programs.
Second, the Windows experience score is a moving target. Remember that the Windows Experience scroe is not based upon a fixed scale but by its relative position to other hardware at the time of the test. As hardware improves, the top hardware gets the top score. The relative scores of hardware will go down as time progresses.
If you are perceiving an actual decline in performance then you should look at:
Updating to the latest driver/applying any firmware updates
Scanning your system for malware
Performing some routine maintenance on your system to defragment the hard drive and remove any un-needed files.