diff --git a/Performance-Tips.md b/Performance-Tips.md
index 06485f0..4ca9a41 100644
--- a/Performance-Tips.md
+++ b/Performance-Tips.md
@@ -31,3 +31,11 @@ B.DrawImage(A, op)
 # Avoid using the screen as a render source
 
 The screen is a special image because the image is cleared at every frame. As explained above, Ebiten records a drawing function calls but using the screen  as a render source makes the calculation complicated.
+
+# Don't call (*Image).ReplacePixels too much
+
+ReplacePixels is a relatively heavy function that calls glTexSubImage2D internally.
+
+# Don't call (*Image).At too much
+
+At is also heavy that tries to solve all the queued draw commands.