diff --git a/TODO b/TODO
index bc90f0e7a020d626e1ab25890e62c16babbb8527..6214f14b62f1f659503761b33a9be7dfc222126b 100644
--- a/TODO
+++ b/TODO
@@ -23,3 +23,5 @@ On screen 1, it will work if fullscreen is turned off, the window is rendered
 and moved so that its top corner is only on screen 1, and then fullscreen is
 turned on (after that, it'll work for all images in the slideshow).
 Maybe I'll someday find out how to fix that :>
+
+The thumbnail code seems to be a bit inefficient with large filelists.