diff --git a/README.md b/README.md
index c1c891a3fbfbec748ec46787b75a10a6b91d4b47..c1ef591ad7b4af4d8b9f810f05aee67fd3722ce6 100644
--- a/README.md
+++ b/README.md
@@ -13,6 +13,9 @@ Typical sequence:
 
 Note: Even 3x fft upscale with 3x rgb downscale make smoother look due to how fft conversion works.
 
+Warning:
+FFT resynth performs very slow if tmpdir is located on slow media. Placing it in tmpfs gives best speed, but may be risky, as imagemagick can easily run out of memory for too big images (I would usually interrupt it before system becomes unresponsive, forcing me to do it anyway).
+
 Requirements:
 - imagemagick
 - coreutils