diff --git a/src/calibre/db/cache.py b/src/calibre/db/cache.py
index 10a9c48625..d28d1f4558 100644
--- a/src/calibre/db/cache.py
+++ b/src/calibre/db/cache.py
@@ -947,7 +947,8 @@ class Cache:
 
     @read_api
     def get_item_id(self, field, item_name):
-        ' Return the item id for item_name (case-insensitive) or None if not found '
+        '''' Return the item id for item_name (case-insensitive) or None if not found.
+        This function is very slow if doing lookups for multiple names use either get_item_ids() or get_item_name_map(). '''
         q = icu_lower(item_name)
         try:
             for item_id, item_val in self.fields[field].table.id_map.items():