My guess would be the registry or temp files. If the registry is full of garbage (which happens over time, some programs don't clean up even when uninstalled) it will slow a lot of things down. The temp folder gets crowded easily as well if a program doesn't use it correctly. When you create and use a temporary file, it is supposed to be flagged so that the system automatically deletes the file when the application terminates (expectedly or otherwise).
The current version of VDJ does this correctly I believe, but I know I did some further work in the new version that implicitly deletes temp files on close, and the uninstaller cleans all VDJ-related registry entries (except for the license key, which you can wipe on reinstall if desired).
Glad you got it figured out Fabio! I've been using CCleaner for years, it's on my keychain flash drive with my password recovery tools, AVG free, etc. You know, for those times you're at a relative or friend's house and get the inevitable "Can you take a look at my computer?"...
Regarding the temp folder, note that any single folder with too many files will cause Windows to be very slow to access. It is exponentially proportionate to the number of files within the folder. Notice that while indexing songs, it will be slower on folders containing many individual files - nothing I can do to speed that up, it's a general filesystem thing. Unfortunately the temp folder is shared among many other programs, so if it gets full of thousands of files, it will be slow when creating/accessing any file within that folder.
PS - I'm going to look into that part of the code and see exactly what part is slowing down when you add a new song... I don't think it has anything to do with the singers.dat file itself, but something I'm doing internally when a new song is added...