diff --git a/doc/060_forget.rst b/doc/060_forget.rst
index 823264906..7222c1711 100644
--- a/doc/060_forget.rst
+++ b/doc/060_forget.rst
@@ -23,6 +23,13 @@ data that was referenced by the snapshot from the repository. This can
 be automated with the ``--prune`` option of the ``forget`` command,
 which runs ``prune`` automatically if snapshots have been removed.
 
+.. Warning::
+
+   Pruning snapshots can be a very time-consuming process, taking nearly
+   as long as backups themselves. During a prune operation, the index is
+   locked and backups cannot be completed. Performance improvements are 
+   planned for this feature.
+
 It is advisable to run ``restic check`` after pruning, to make sure
 you are alerted, should the internal data structures of the repository
 be damaged.