>>434 ドキュメントにそれっぽいことが書いてあるけど
https://hackage.haskell.org/package/containers-0.6.3.1/docs/Data-Map-Strict.html

> The implementation of Map is based on size balanced binary trees (or trees of bounded balance) as described by:

> If you don't care about ordering, consider use Data.HashMap.Strict from the unordered-containers package instead.