diff --git a/USER_GUIDE.md b/USER_GUIDE.md
index 6975e944..f3139521 100644
--- a/USER_GUIDE.md
+++ b/USER_GUIDE.md
@@ -230,6 +230,7 @@ Accessible by pressing **Confirm** while inside a book.
 Please note that this firmware is currently in active development. The following features are **not yet supported** but are planned for future updates:
 
 * **Images:** Embedded images in e-books will not render.
+* **Cover Images:** Large cover images embedded into EPUB require several seconds (~10s for ~2000 pixel tall image) to convert for sleep screen and home screen thumbnail. Consider optimizing the EPUB with e.g. https://github.com/bigbag/epub-to-xtc-converter to speed this up.
 
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