Added note on optimization

Change-Id: I1c20452e6f320ee27859d2d18d56138871e09dde
diff --git a/script/korapxml2krill b/script/korapxml2krill
index 6b7b9df..97a6918 100644
--- a/script/korapxml2krill
+++ b/script/korapxml2krill
@@ -467,6 +467,9 @@
       # Create temporary file
       $temp = File::Temp->newdir;
 
+      # TODO: Check if $filename exist at the beginning,
+      # because extraction can be horrible slow!
+
       # Extract from archive
       if ($archive->extract($dirs[$i], $temp)) {