diff --git a/src/google/protobuf/extension_set.cc b/src/google/protobuf/extension_set.cc
index 3ba79d6bde45..461993dba086 100644
--- a/src/google/protobuf/extension_set.cc
+++ b/src/google/protobuf/extension_set.cc
@@ -1239,9 +1239,9 @@ uint8_t* ExtensionSet::_InternalSerializeImpl(
     return target;
   }
   const KeyValue* end = flat_end();
-  for (const KeyValue* it = std::lower_bound(
-           flat_begin(), end, start_field_number, KeyValue::FirstComparator());
-       it != end && it->first < end_field_number; ++it) {
+  const KeyValue* it = flat_begin();
+  while (it != end && it->first < start_field_number) ++it;
+  for (; it != end && it->first < end_field_number; ++it) {
     target = it->second.InternalSerializeFieldWithCachedSizesToArray(
         extendee, this, it->first, target, stream);
   }
@@ -1566,9 +1566,11 @@ const ExtensionSet::Extension* ExtensionSet::FindOrNull(int key) const {
   if (flat_size_ == 0) {
     return nullptr;
   } else if (PROTOBUF_PREDICT_TRUE(!is_large())) {
-    auto it = std::lower_bound(flat_begin(), flat_end() - 1, key,
-                               KeyValue::FirstComparator());
-    return it->first == key ? &it->second : nullptr;
+    for (auto it = flat_begin(), end = flat_end();
+         it != end && it->first <= key; ++it) {
+      if (it->first == key) return &it->second;
+    }
+    return nullptr;
   } else {
     return FindOrNullInLargeMap(key);
   }
@@ -1601,10 +1603,9 @@ std::pair<ExtensionSet::Extension*, bool> ExtensionSet::Insert(int key) {
     return {&maybe.first->second, maybe.second};
   }
   KeyValue* end = flat_end();
-  KeyValue* it =
-      std::lower_bound(flat_begin(), end, key, KeyValue::FirstComparator());
-  if (it != end && it->first == key) {
-    return {&it->second, false};
+  KeyValue* it = flat_begin();
+  for (; it != end && it->first <= key; ++it) {
+    if (it->first == key) return {&it->second, false};
   }
   if (flat_size_ < flat_capacity_) {
     std::copy_backward(it, end, end + 1);
@@ -1681,11 +1682,12 @@ void ExtensionSet::Erase(int key) {
     return;
   }
   KeyValue* end = flat_end();
-  KeyValue* it =
-      std::lower_bound(flat_begin(), end, key, KeyValue::FirstComparator());
-  if (it != end && it->first == key) {
-    std::copy(it + 1, end, it);
-    --flat_size_;
+  for (KeyValue* it = flat_begin(); it != end && it->first <= key; ++it) {
+    if (it->first == key) {
+      std::copy(it + 1, end, it);
+      --flat_size_;
+      return;
+    }
   }
 }
 
diff --git a/src/google/protobuf/extension_set.h b/src/google/protobuf/extension_set.h
index 2d8e7530294d..ab4b0f92edfb 100644
--- a/src/google/protobuf/extension_set.h
+++ b/src/google/protobuf/extension_set.h
@@ -725,27 +725,17 @@ class PROTOBUF_EXPORT ExtensionSet {
     const FieldDescriptor* descriptor;
   };
 
-  // The Extension struct is small enough to be passed by value, so we use it
-  // directly as the value type in mappings rather than use pointers.  We use
+  // The Extension struct is small enough to be passed by value so we use it
+  // directly as the value type in mappings rather than use pointers. We use
   // sorted maps rather than hash-maps because we expect most ExtensionSets will
-  // only contain a small number of extension.  Also, we want AppendToList and
-  // deterministic serialization to order fields by field number.
+  // only contain a small number of extensions, and we want AppendToList and
+  // deterministic serialization to order fields by field number. In flat mode,
+  // the number of elements is small enough that linear search is faster than
+  // binary search.
 
   struct KeyValue {
     int first;
     Extension second;
-
-    struct FirstComparator {
-      bool operator()(const KeyValue& lhs, const KeyValue& rhs) const {
-        return lhs.first < rhs.first;
-      }
-      bool operator()(const KeyValue& lhs, int key) const {
-        return lhs.first < key;
-      }
-      bool operator()(int key, const KeyValue& rhs) const {
-        return key < rhs.first;
-      }
-    };
   };
 
   using LargeMap = absl::btree_map<int, Extension>;