diff --git a/Annex/RepoSize.hs b/Annex/RepoSize.hs
index 6e823c98b9..dac089a962 100644
--- a/Annex/RepoSize.hs
+++ b/Annex/RepoSize.hs
@@ -5,6 +5,8 @@
  - Licensed under the GNU AGPL version 3 or higher.
  -}
 
+{-# LANGUAGE OverloadedStrings #-}
+
 module Annex.RepoSize (
 	getRepoSizes,
 ) where
@@ -43,6 +45,7 @@ updateRepoSizes = bracket Db.openDb Db.closeDb $ \h -> do
 					calculatefromscratch h >>= set
   where
 	calculatefromscratch h = do
+		showSideAction "calculating repository sizes"
 		(sizemap, branchsha) <- calcBranchRepoSizes
 		liftIO $ Db.setRepoSizes h sizemap branchsha
 		journalledRepoSizes sizemap branchsha
diff --git a/doc/git-annex-preferred-content.mdwn b/doc/git-annex-preferred-content.mdwn
index 8069cb201f..7ef98d14c8 100644
--- a/doc/git-annex-preferred-content.mdwn
+++ b/doc/git-annex-preferred-content.mdwn
@@ -294,6 +294,14 @@ elsewhere to allow removing it).
   expression, which will make repositories want to move files around as
   necessary in order to get fully balanced.
 
+  Using this in a perferred content expression makes git-annex need to do
+  some additional work to keep track of how full repositories are. Usually
+  that won't affect performance much. However, the first time git-annex
+  processes this in a given git repository, it will need to examine
+  all the locations of all files, which can be slow when there are a lot of
+  them. When this causes git-annex to do a lot of work, it will
+  display "(calculating repository sizes)".
+
   Note that `not balanced` is a bad thing to put in a preferred content
   expression for the same reason `not present` is.