diff --git a/src/generate.js b/src/generate.js
index 6db7a9a..7d46912 100644
--- a/src/generate.js
+++ b/src/generate.js
@@ -284,11 +284,13 @@ export const generateCSSRuleset = (
         })
     );
 
-    const rules = prefixedRules.map(([key, value]) => {
-        const stringValue = stringifyValue(key, value);
-        const ret = `${kebabifyStyleName(key)}:${stringValue};`;
-        return useImportant === false ? ret : importantify(ret);
-    }).join("");
+    const transformValue = (useImportant === false)
+        ? stringifyValue
+        : (key, value) => importantify(stringifyValue(key, value));
+
+    const rules = prefixedRules
+        .map(([key, value]) => `${kebabifyStyleName(key)}:${transformValue(key, value)};`)
+        .join("");
 
     if (rules) {
         return `${selector}{${rules}}`;
diff --git a/src/util.js b/src/util.js
index ca5ab72..0e24817 100644
--- a/src/util.js
+++ b/src/util.js
@@ -215,11 +215,15 @@ function murmurhash2_32_gc(str) {
 export const hashObject = (object /* : ObjectMap */) /* : string */ => murmurhash2_32_gc(JSON.stringify(object));
 
 
-const IMPORTANT_RE = /^([^:]+:.*?)( !important)?;$/;
-
-// Given a single style rule string like "a: b;", adds !important to generate
-// "a: b !important;".
-export const importantify = (string /* : string */) /* : string */ =>
-    string.replace(
-        IMPORTANT_RE,
-        (_, base) => base + " !important;");
+// Given a single style value string like the "b" from "a: b;", adds !important
+// to generate "b !important".
+export const importantify = (string /* : string */) /* : string */ => (
+    // Bracket string character access is very fast, and in the default case we
+    // normally don't expect there to be "!important" at the end of the string
+    // so we can use this simple check to take an optimized path. If there
+    // happens to be a "!" in this position, we follow up with a more thorough
+    // check.
+    (string[string.length - 10] === '!' && string.slice(-11) === ' !important')
+        ? string
+        : `${string} !important`
+);