diff --git a/is.ts b/is.ts
index 4b72d00..1ee9fb0 100644
--- a/is.ts
+++ b/is.ts
@@ -138,8 +138,13 @@ export function isRecord(
 export function isRecordOf<T>(
   pred: Predicate<T>,
 ): Predicate<RecordOf<T>> {
-  return (x: unknown): x is RecordOf<T> =>
-    isRecord(x) && Object.values(x).every(pred);
+  return (x: unknown): x is RecordOf<T> => {
+    if (!isRecord(x)) return false;
+    for (const k in x) {
+      if (!pred(x[k])) return false;
+    }
+    return true;
+  };
 }
 
 type FlatType<T> = T extends RecordOf<unknown>
@@ -188,16 +193,35 @@ export function isObjectOf<
   predObj: T,
   options: { strict?: boolean } = {},
 ): Predicate<ObjectOf<T>> {
-  const preds = Object.entries(predObj);
-  const allKeys = new Set(preds.map(([key]) => key));
-  const requiredKeys = preds
-    .filter(([_, pred]) => !(pred as OptionalPredicate<unknown>).optional)
-    .map(([key]) => key);
-  const hasKeys = options.strict
-    ? (props: string[]) => props.every((p) => allKeys.has(p))
-    : (props: string[]) => requiredKeys.every((k) => props.includes(k));
-  return (x: unknown): x is ObjectOf<T> =>
-    isRecord(x) && hasKeys(Object.keys(x)) && preds.every(([k, p]) => p(x[k]));
+  return options.strict ? isObjectOfStrict(predObj) : isObjectOfLoose(predObj);
+}
+
+function isObjectOfLoose<
+  T extends RecordOf<Predicate<unknown>>,
+>(
+  predObj: T,
+): Predicate<ObjectOf<T>> {
+  return (x: unknown): x is ObjectOf<T> => {
+    if (!isRecord(x)) return false;
+    for (const k in predObj) {
+      if (!predObj[k](x[k])) return false;
+    }
+    return true;
+  };
+}
+
+function isObjectOfStrict<
+  T extends RecordOf<Predicate<unknown>>,
+>(
+  predObj: T,
+): Predicate<ObjectOf<T>> {
+  const keys = new Set(Object.keys(predObj));
+  const pred = isObjectOfLoose(predObj);
+  return (x: unknown): x is ObjectOf<T> => {
+    if (!pred(x)) return false;
+    const ks = Object.keys(x);
+    return ks.length <= keys.size && ks.every((k) => keys.has(k));
+  };
 }
 
 /**
@@ -366,9 +390,12 @@ export type OptionalPredicate<T> = Predicate<T | undefined> & {
 export function isOptionalOf<T>(
   pred: Predicate<T>,
 ): OptionalPredicate<T> {
-  return Object.assign(isOneOf([isUndefined, pred]), {
-    optional: true as const,
-  });
+  return Object.assign(
+    (x: unknown): x is Predicate<T | undefined> => isUndefined(x) || pred(x),
+    {
+      optional: true as const,
+    },
+  ) as OptionalPredicate<T>;
 }
 
 export default {
diff --git a/is_bench.ts b/is_bench.ts
index 2867d6c..06aba3f 100644
--- a/is_bench.ts
+++ b/is_bench.ts
@@ -170,6 +170,15 @@ Deno.bench({
     }
   },
 });
+Deno.bench({
+  name: "is.ObjectOf (strict)",
+  fn: () => {
+    const pred = is.ObjectOf(predObj, { strict: true });
+    for (const c of cs) {
+      pred(c);
+    }
+  },
+});
 
 const isObjectOfPred = is.ObjectOf(predObj);
 Deno.bench({
@@ -181,6 +190,16 @@ Deno.bench({
   },
 });
 
+const isObjectOfStrictPred = is.ObjectOf(predObj, { strict: true });
+Deno.bench({
+  name: "is.ObjectOf (pre, strict)",
+  fn: () => {
+    for (const c of cs) {
+      isObjectOfStrictPred(c);
+    }
+  },
+});
+
 Deno.bench({
   name: "is.Function",
   fn: () => {
diff --git a/is_test.ts b/is_test.ts
index 0649545..e5ecfc4 100644
--- a/is_test.ts
+++ b/is_test.ts
@@ -280,7 +280,7 @@ Deno.test("isObjectOf<T>", async (t) => {
   });
   await testWithExamples(
     t,
-    isObjectOf({ a: (_: unknown): _ is unknown => true }),
+    isObjectOf({ a: (_: unknown): _ is unknown => false }),
     { excludeExamples: ["record"] },
   );
   await t.step("with optional properties", async (t) => {