added note on performance

docs/hooks/hook_civicrm_alterReportVar.md

@@ -20,6 +20,12 @@ This hook is used to add or modify display columns and filters.
 
 -   null
 
+## Note
+
+It is often more performant to change the report query on $varType == 'sql' than
+to do database lookups on each row in the rows in the $var array on 
+$varType == 'rows'. 
+
 ## Example
 
 From the [Mandrill Transaction
@@ -92,4 +98,4 @@ columns appropriately.
           }
         }
       }
-    }
\ No newline at end of file
+    }