Simply remove (or rename) the Data/Reflection directory, so that Sandcastle cannot find the information about .NET Framework libraries. You will loose the ability to have MSDN help on .NET Framework classes links in your documentation, but I think this is not crucial for day-to-day builds, compared to the substantial increase in your build speed. And also, by deleting the contents of the Reflection folder you’ll gain some 200 MB of space on your disk.