diff --git a/examples/SimpleServer/SimpleServer.ino b/examples/SimpleServer/SimpleServer.ino
index 2119242..fd53639 100644
--- a/examples/SimpleServer/SimpleServer.ino
+++ b/examples/SimpleServer/SimpleServer.ino
@@ -73,8 +73,11 @@ void setup() {
   server.on("/download", HTTP_HEAD | HTTP_GET, [](AsyncWebServerRequest* request) {
     if (request->method() == HTTP_HEAD) {
       AsyncWebServerResponse* response = request->beginResponse(200, "application/octet-stream");
-      response->setContentLength(1024); // myFile.getSize()
-      response->addHeader("Accept-Ranges", "bytes");
+      response->addHeader(asyncsrv::T_Accept_Ranges, "bytes");
+      response->addHeader(asyncsrv::T_Content_Length, 10);
+      response->setContentLength(1024); // overrides previous one
+      response->addHeader(asyncsrv::T_Content_Type, "foo");
+      response->setContentType("application/octet-stream"); // overrides previous one
       // ...
       request->send(response);
     } else {
diff --git a/src/AsyncEventSource.cpp b/src/AsyncEventSource.cpp
index 639fd56..30ad271 100644
--- a/src/AsyncEventSource.cpp
+++ b/src/AsyncEventSource.cpp
@@ -392,7 +392,8 @@ AsyncEventSourceResponse::AsyncEventSourceResponse(AsyncEventSource* server) {
 }
 
 void AsyncEventSourceResponse::_respond(AsyncWebServerRequest* request) {
-  String out = _assembleHead(request->version());
+  String out;
+  _assembleHead(out, request->version());
   request->client()->write(out.c_str(), _headLength);
   _state = RESPONSE_WAIT_ACK;
 }
diff --git a/src/AsyncWebSocket.cpp b/src/AsyncWebSocket.cpp
index 88d88ec..a5c8d7b 100644
--- a/src/AsyncWebSocket.cpp
+++ b/src/AsyncWebSocket.cpp
@@ -1192,7 +1192,8 @@ void AsyncWebSocketResponse::_respond(AsyncWebServerRequest* request) {
     request->client()->close(true);
     return;
   }
-  String out(_assembleHead(request->version()));
+  String out;
+  _assembleHead(out, request->version());
   request->client()->write(out.c_str(), _headLength);
   _state = RESPONSE_WAIT_ACK;
 }
diff --git a/src/ESPAsyncWebServer.h b/src/ESPAsyncWebServer.h
index 2d1f100..ff6b2fb 100644
--- a/src/ESPAsyncWebServer.h
+++ b/src/ESPAsyncWebServer.h
@@ -155,7 +155,9 @@ class AsyncWebHeader {
     const String& name() const { return _name; }
     const String& value() const { return _value; }
     String toString() const {
-      String str = _name;
+      String str;
+      str.reserve(_name.length() + _value.length() + 2);
+      str.concat(_name);
       str.concat((char)0x3a);
       str.concat((char)0x20);
       str.concat(_value);
@@ -583,8 +585,19 @@ class AsyncWebServerResponse {
     virtual void setContentType(const char* type);
     virtual bool addHeader(const char* name, const char* value, bool replaceExisting = true);
     bool addHeader(const String& name, const String& value, bool replaceExisting = true) { return addHeader(name.c_str(), value.c_str(), replaceExisting); }
+    bool addHeader(const char* name, long value, bool replaceExisting = true) { return addHeader(name, String(value), replaceExisting); }
+    bool addHeader(const String& name, long value, bool replaceExisting = true) { return addHeader(name.c_str(), value, replaceExisting); }
     virtual bool removeHeader(const char* name);
-    virtual String _assembleHead(uint8_t version);
+    virtual const AsyncWebHeader* getHeader(const char* name) const;
+    
+    [[deprecated("Use instead: _assembleHead(String& buffer, uint8_t version)")]]
+    String _assembleHead(uint8_t version) {
+      String buffer;
+      _assembleHead(buffer, version);
+      return buffer;
+    }
+    virtual void _assembleHead(String& buffer, uint8_t version);
+
     virtual bool _started() const;
     virtual bool _finished() const;
     virtual bool _failed() const;
diff --git a/src/WebRequest.cpp b/src/WebRequest.cpp
index 95fda7e..022911e 100644
--- a/src/WebRequest.cpp
+++ b/src/WebRequest.cpp
@@ -624,7 +624,6 @@ bool AsyncWebServerRequest::hasHeader(const __FlashStringHelper* data) const {
 
 const AsyncWebHeader* AsyncWebServerRequest::getHeader(const char* name) const {
   auto iter = std::find_if(std::begin(_headers), std::end(_headers), [&name](const AsyncWebHeader& header) { return header.name().equalsIgnoreCase(name); });
-
   return (iter == std::end(_headers)) ? nullptr : &(*iter);
 }
 
diff --git a/src/WebResponses.cpp b/src/WebResponses.cpp
index af9c349..e3fbfd1 100644
--- a/src/WebResponses.cpp
+++ b/src/WebResponses.cpp
@@ -125,9 +125,8 @@ const char* AsyncWebServerResponse::responseCodeToString(int code) {
       return T_HTTP_CODE_ANY;
   }
 }
-#else // ESP8266
-const __FlashStringHelper* AsyncWebServerResponse::responseCodeToString(int code)
-{
+#else  // ESP8266
+const __FlashStringHelper* AsyncWebServerResponse::responseCodeToString(int code) {
   switch (code) {
     case 100:
       return FPSTR(T_HTTP_CODE_100);
@@ -230,12 +229,12 @@ void AsyncWebServerResponse::setCode(int code) {
 }
 
 void AsyncWebServerResponse::setContentLength(size_t len) {
-  if (_state == RESPONSE_SETUP)
+  if (_state == RESPONSE_SETUP && addHeader(T_Content_Length, len, true))
     _contentLength = len;
 }
 
 void AsyncWebServerResponse::setContentType(const char* type) {
-  if (_state == RESPONSE_SETUP)
+  if (_state == RESPONSE_SETUP && addHeader(T_Content_Type, type, true))
     _contentType = type;
 }
 
@@ -249,6 +248,11 @@ bool AsyncWebServerResponse::removeHeader(const char* name) {
   return false;
 }
 
+const AsyncWebHeader* AsyncWebServerResponse::getHeader(const char* name) const {
+  auto iter = std::find_if(std::begin(_headers), std::end(_headers), [&name](const AsyncWebHeader& header) { return header.name().equalsIgnoreCase(name); });
+  return (iter == std::end(_headers)) ? nullptr : &(*iter);
+}
+
 bool AsyncWebServerResponse::addHeader(const char* name, const char* value, bool replaceExisting) {
   for (auto i = _headers.begin(); i != _headers.end(); ++i) {
     if (i->name().equalsIgnoreCase(name)) {
@@ -268,41 +272,56 @@ bool AsyncWebServerResponse::addHeader(const char* name, const char* value, bool
   return true;
 }
 
-String AsyncWebServerResponse::_assembleHead(uint8_t version) {
+void AsyncWebServerResponse::_assembleHead(String& buffer, uint8_t version) {
   if (version) {
     addHeader(T_Accept_Ranges, T_none, false);
     if (_chunked)
       addHeader(T_Transfer_Encoding, T_chunked, false);
   }
-  String out;
-  constexpr size_t bufSize = 300;
-  char buf[bufSize];
 
-#ifndef ESP8266
-  snprintf(buf, bufSize, "HTTP/1.%d %d %s\r\n", version, _code, responseCodeToString(_code));
-#else
-  snprintf_P(buf, bufSize, PSTR("HTTP/1.%d %d %s\r\n"), version, _code, String(responseCodeToString(_code)).c_str());
-#endif
-  out.concat(buf);
+  if (_sendContentLength)
+    addHeader(T_Content_Length, String(_contentLength), false);
 
-  if (_sendContentLength) {
-    snprintf_P(buf, bufSize, PSTR("Content-Length: %d\r\n"), _contentLength);
-    out.concat(buf);
-  }
-  if (_contentType.length()) {
-    snprintf_P(buf, bufSize, PSTR("Content-Type: %s\r\n"), _contentType.c_str());
-    out.concat(buf);
-  }
+  if (_contentType.length())
+    addHeader(T_Content_Type, _contentType.c_str(), false);
+
+  // precompute buffer size to avoid reallocations by String class
+  size_t len = 0;
+  len += 50; // HTTP/1.1 200 <reason>\r\n
+  for (const auto& header : _headers)
+    len += header.name().length() + header.value().length() + 4;
 
+  // prepare buffer
+  buffer.reserve(len);
+
+  // HTTP header
+#ifdef ESP8266
+  buffer.concat(PSTR("HTTP/1."));
+#else
+  buffer.concat("HTTP/1.");
+#endif
+  buffer.concat(version);
+  buffer.concat(' ');
+  buffer.concat(_code);
+  buffer.concat(' ');
+  buffer.concat(responseCodeToString(_code));
+  buffer.concat(T_rn);
+
+  // Add headers
   for (const auto& header : _headers) {
-    snprintf_P(buf, bufSize, PSTR("%s: %s\r\n"), header.name().c_str(), header.value().c_str());
-    out.concat(buf);
+    buffer.concat(header.name());
+#ifdef ESP8266
+    buffer.concat(PSTR(": "));
+#else
+    buffer.concat(": ");
+#endif
+    buffer.concat(header.value());
+    buffer.concat(T_rn);
   }
   _headers.clear();
 
-  out.concat(T_rn);
-  _headLength = out.length();
-  return out;
+  buffer.concat(T_rn);
+  _headLength = buffer.length();
 }
 
 bool AsyncWebServerResponse::_started() const { return _state > RESPONSE_SETUP; }
@@ -337,7 +356,8 @@ AsyncBasicResponse::AsyncBasicResponse(int code, const char* contentType, const
 
 void AsyncBasicResponse::_respond(AsyncWebServerRequest* request) {
   _state = RESPONSE_HEADERS;
-  String out = _assembleHead(request->version());
+  String out;
+  _assembleHead(out, request->version());
   size_t outLen = out.length();
   size_t space = request->client()->space();
   if (!_contentLength && space >= outLen) {
@@ -411,7 +431,7 @@ AsyncAbstractResponse::AsyncAbstractResponse(AwsTemplateProcessor callback) : _c
 
 void AsyncAbstractResponse::_respond(AsyncWebServerRequest* request) {
   addHeader(T_Connection, T_close, false);
-  _head = _assembleHead(request->version());
+  _assembleHead(_head, request->version());
   _state = RESPONSE_HEADERS;
   _ack(request, 0, 0);
 }
@@ -635,9 +655,9 @@ AsyncFileResponse::~AsyncFileResponse() {
 void AsyncFileResponse::_setContentTypeFromPath(const String& path) {
 #if HAVE_EXTERN_GET_Content_Type_FUNCTION
   #ifndef ESP8266
-    extern const char* getContentType(const String& path);
+  extern const char* getContentType(const String& path);
   #else
-    extern const __FlashStringHelper* getContentType(const String& path);
+  extern const __FlashStringHelper* getContentType(const String& path);
   #endif
   _contentType = getContentType(path);
 #else