It seems that we finally managed to solve this issue.
After several hours in searching for a solution I've discovered a critical misundestanding:
In the download page the libraries are reported with two different target cpu version. x86 and x64.
My development workstation has Windows 7 x64 with an Intel I3 CPU so at the beginning I adopted the x64 version of the libraries, but checking into the GAC I realized that those assemblies (v.126.96.36.199) figured as A64 architecture, instead of the previous
version (v.10.0.0.0) that figured as MSIL architecture. This caused my application to crash with the classic "the assembly xxx is missing" message.
Then I downloaded and installed the x86 version and this time the application did not crashed anymore but I experienced the slow script execution.
At the end I removed all the x64 version, that were probably conficting with the others, and the issue is solved.
Marked As Answer bySassatokieroThursday, April 05, 2012 10:44 PM