diff --git a/TODO b/TODO
index 6214f14b62f1f659503761b33a9be7dfc222126b..af1f3c63b24a72951df4f5af3e99ed69405e9193 100644
--- a/TODO
+++ b/TODO
@@ -24,4 +24,7 @@ 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.
+Since the thumbnail mode draws an image, it gets kinda slow once you have lots
+of files in the filelist (->big thumbnail index image). The ideal way would be
+to rewrite it to use some GUI-ish thing, alternatively a workaround (only
+rendering every n'th image?) would be nice.