Update README with more info on why (not) to use this

README.md

@@ -13,9 +13,11 @@ querystrings. This crate is designed to extend [`serde_urlencoded`][urlencoded]
 when using nested parameters, similar to those used by [qs][qs] for Node, and
 commonly used by Ruby on Rails via [Rack][Rack].
 
-The core of the library was inspired by
-[`serde_urlencoded`][urlencoded], which should be preferred
-over this crate whenever non-nested query parameters are sufficient. It is built
+The core of the library was inspired by [`serde_urlencoded`][urlencoded].
+In order to support abitrarily nested structs encoded in arbitrary orders, we
+perform two passes over the input string. This likely adds a non-trivial amount
+of memory and compute. Due to this `serde_urlencoded` should be preferred
+over this crate whenever non-nested query parameters are sufficient. The crate is built
 upon [Serde], a high performance generic serialization framework and [rust-url],
 a URL parser for Rust.