diff --git a/src/ch04/s00-concerns.md b/src/ch04/s00-concerns.md
index fc56360..2c3c367 100644
--- a/src/ch04/s00-concerns.md
+++ b/src/ch04/s00-concerns.md
@@ -15,4 +15,7 @@ See the [relevant subchapter](./s04-covariance.md)
 ## `Fail`/`FailMonad`/etc. have unclear semantics
 See the [relevant subchapter](./s05-fail.md)
 
+## Is `FutureInstance` actually feasible?
+Todo: measure `futures::join` performance.
+
 ## Can `WeakFunctor` be an associated type of a `Functor` instead of its supertype?