diff --git a/crates/ruff_linter/src/rules/pyupgrade/rules/use_pep604_isinstance.rs b/crates/ruff_linter/src/rules/pyupgrade/rules/use_pep604_isinstance.rs
index af3e40687a7c4a..4849dda1854915 100644
--- a/crates/ruff_linter/src/rules/pyupgrade/rules/use_pep604_isinstance.rs
+++ b/crates/ruff_linter/src/rules/pyupgrade/rules/use_pep604_isinstance.rs
@@ -39,9 +39,13 @@ impl CallKind {
 ///
 /// ## Why is this bad?
 /// Since Python 3.10, `isinstance` and `issubclass` can be passed a
-/// `|`-separated union of types, which is more concise and consistent
+/// `|`-separated union of types, which is consistent
 /// with the union operator introduced in [PEP 604].
 ///
+/// Note that this results in slower code. Ignore this rule if the
+/// performance of an `isinstance` or `issubclass` check is a
+/// concern, e.g., in a hot loop.
+///
 /// ## Example
 /// ```python
 /// isinstance(x, (int, float))