SSD very slow after installing some programs

I´m having a problem with an internal 2,5" Verbatim 128GB SATAII SSD. From the moment I installed it on my computer I had to deal with a poor performance, but I immensely improved it by switching to AHCI and making a Secure Erase with Parted Magic (as other posts in this forum explain).

Right after installing a fresh copy of Windows 8 and some drivers from Dell and NVIDIA, I had AS SSD Benchmark overall score of 228 (Seq. 244,09MBs Read / 140,55MBs Write, 4K 10,46MBs Read / 23,81MBs Write). Apparently quite OK for a SATA II drive.

However, after installing Office 2010 Home and some minor programs, such as Epson Scan, CCleaner, iTunes and Dropbox, its performance went down drastically. AS SSD Benchmark had an overall score of just 47 (Seq. 83,93MBs Read / 32,51MBs Write, 4K 4,64MBs Read / 2,56MBs Write).

Currently, all the system and programs take only 35GB of SSD´s space. I wonder what will happen when I install 3D modelling software and Adobe Creative Suite.

I made a similar test on Windows 7 and the same fall in performance happens.

Is that normal? Is there a way to improve this?

Henri