Help untagged users know of performance issues and fix them

Inspired by serde-rs/serde#2101

Author: epage

_src/container-attrs.md

@@ -72,6 +72,11 @@
 
   When no variant matches, the error may be uninformative which can be improved with [`serde(expecting)`](#expecting).
 
+  In performance-critical code, checking each variant and processing the errors can be slow.
+  In these cases, it may be better to hand-implement the deserialize trait for which [serde-untagged] may help.
+
+  [serde-untagged]: https://docs.rs/serde-untagged
+
 - ##### `#[serde(bound = "T: MyTrait")]` {#bound}
 
   Where-clause for the `Serialize` and `Deserialize` impls. This replaces any