diff --git a/CHANGES/9603.misc.rst b/CHANGES/9603.misc.rst
new file mode 100644
index 00000000000..8a27657cdb9
--- /dev/null
+++ b/CHANGES/9603.misc.rst
@@ -0,0 +1 @@
+Improved performance of serializing HTTP headers -- by :user:`bdraco`.
diff --git a/aiohttp/_http_writer.pyx b/aiohttp/_http_writer.pyx
index eff85219586..d19c20d76cc 100644
--- a/aiohttp/_http_writer.pyx
+++ b/aiohttp/_http_writer.pyx
@@ -127,10 +127,6 @@ def _serialize_headers(str status_line, headers):
 
     _init_writer(&writer)
 
-    for key, val in headers.items():
-        _safe_header(to_str(key))
-        _safe_header(to_str(val))
-
     try:
         if _write_str(&writer, status_line) < 0:
             raise
@@ -140,6 +136,9 @@ def _serialize_headers(str status_line, headers):
             raise
 
         for key, val in headers.items():
+            _safe_header(to_str(key))
+            _safe_header(to_str(val))
+
             if _write_str(&writer, to_str(key)) < 0:
                 raise
             if _write_byte(&writer, b':') < 0: