What the "Collecting maps" process does, is to search recursively in all Library paths, for folders with name "maps".
Next, these folders are added to Customize->User Paths->External files. This is required so Forest can locate the library textures automatically, without you have to add all "maps" manually.
With the default libraries installed on the local drive, the checking process should not take more than a few seconds.
But if you have many library paths, with tons of folders within them, the process may take a while to respond.
It may even worse if you use Network paths, because there is a delay to locate and open each folder.
And if some of the paths is not accessible (for example, pointing to a missing network node), Windows may freeze some time until it reaches the timeout.
Unfortunately, since this problem depends exclusively of each particular Library configuration and infraestructure, there is not much we can do.
If you are experiencing this issue, we suggest to disable the collecting maps feature, as suggested above.
The folders in which Forest searches the maps, are those defined in Library Browser->Preferences->Paths.