Update README.md

README.md

@@ -87,6 +87,13 @@ func main() {
 }
 ```
 
+Caveats
+-------
+
+* Optimized access routines are used for `image.RGBA`, `image.RGBA64`, `image.YCbCr`, `image.Gray`, and `image.Gray16` types. All other image types are accessed in a generic way that will result in slow processing speed.
+* JPEG images are stored in `image.YCbCr`. This image format stores data in a way that will decrease processing speed. A resize may be up to 2 times slower than with `image.RGBA`. 
+
+
 Downsizing Samples
 -------