diff --git a/README.md b/README.md
index a6bed7c..1164b56 100644
--- a/README.md
+++ b/README.md
@@ -14,11 +14,19 @@ The implementation of `HashMap` is based on "Optimizing Hash-Array
 Mapped Tries for Fast and Lean Immutable JVM Collections" (Steindorfer
 and Vinju, OOPSLA 2015, https://doi.org/10.1145/2814270.2814312).
 
+Performance
+-----------
+
+HashMaps are fast. Most common operations are 1.5x-3x faster than the
+`Ord`-based map. Very limited testing puts `insert` and `lookup` at
+around 10% faster than immutable.js.
+
 Related
 -------
 
 - [purescript-ordered-collections](https://github.com/purescript/purescript-ordered-collections): ordered collections in PureScript
 - [unordered-containers](http://hackage.haskell.org/package/unordered-containers): efficient hashing-based container types in Haskell
+- [immutable.js](https://facebook.github.io/immutable-js/): immutable collections for JavaScript and TypeScript
 
 Documentation
 -------------