diff --git a/.changeset/shy-tools-rest.md b/.changeset/shy-tools-rest.md
new file mode 100644
index 0000000000..881f4d1e7f
--- /dev/null
+++ b/.changeset/shy-tools-rest.md
@@ -0,0 +1,5 @@
+---
+"stylelint": patch
+---
+
+Fixed: `function-no-unknown` performance
diff --git a/lib/rules/function-no-unknown/index.js b/lib/rules/function-no-unknown/index.js
index 383edbc68b..92aa5ed753 100644
--- a/lib/rules/function-no-unknown/index.js
+++ b/lib/rules/function-no-unknown/index.js
@@ -62,6 +62,8 @@ const rule = (primary, secondaryOptions) => {
 		root.walkDecls((decl) => {
 			const { value } = decl;
 
+			if (!value.includes('(')) return;
+
 			if (!isStandardSyntaxValue(value)) return;
 
 			/**