VFS is fully scanned

hi,
i created my own VFS implementation and added it to the PSI by using a 'PsiDirectoryImpl' with the root virtual file of my VFS. This works great with one drawback: as soon as i open one subdirectory, the entire VFS is scanned, leading eventually to "Out of head space".

How can i prevent this from happen, so that only the direct children of a directory are created when i navigate to the tree?

regards, toby