All include files appear to the ASP processor as belonging to the main ASP file, therefore they are read in memory and parsed as if they were physically pasted into the main ASP file. Therefore, while you should use include files for holding commonly used routines, you should be aware that the longer they are, the most adverse impact they have on performance.

For this reason you should use many smaller include files, rather than few large include files that contain all the routines you use in your ASP application. For the same reason, you should avoid nested include files, or limit the nesting level to 2 level at maximum.