FAQ: Why are backups slow on a Linux server that is a member of a windows domain?

backport to 1.4-maint

docs/faq.rst

@@ -489,6 +489,13 @@ using ``nfs=nostale_ro``. Doing so implies mounting the filesystem read-only.
 Another option is to not consider inode numbers in the files cache by passing
 ``--files-cache=ctime,size``.
 
+Why are backups slow on a Linux server that is a member of a Windows domain?
+----------------------------------------------------------------------------
+
+If a Linux server is a member of a Windows domain, username to userid resolution might be 
+performed via ``winbind`` without caching, which can slow down backups significantly. 
+You can use e.g. ``nscd`` to add caching and improve the speed.
+
 Security
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