diff --git a/src/openai/_base_client.py b/src/openai/_base_client.py
index 58490e4430..dd7ff7d176 100644
--- a/src/openai/_base_client.py
+++ b/src/openai/_base_client.py
@@ -698,30 +698,31 @@ def _parse_retry_after_header(self, response_headers: Optional[httpx.Headers] =
         if response_headers is None:
             return None
 
+        # Prefer direct item lookup: Headers.__getitem__ and __contains__ are O(1) and avoid extra string normalization
         # First, try the non-standard `retry-after-ms` header for milliseconds,
         # which is more precise than integer-seconds `retry-after`
-        try:
-            retry_ms_header = response_headers.get("retry-after-ms", None)
-            return float(retry_ms_header) / 1000
-        except (TypeError, ValueError):
-            pass
+        if "retry-after-ms" in response_headers:
+            retry_ms_header = response_headers["retry-after-ms"]
+            try:
+                # If empty/None/invalid, still catch it quick
+                return float(retry_ms_header) / 1000
+            except (TypeError, ValueError):
+                pass
 
         # Next, try parsing `retry-after` header as seconds (allowing nonstandard floats).
-        retry_header = response_headers.get("retry-after")
-        try:
-            # note: the spec indicates that this should only ever be an integer
-            # but if someone sends a float there's no reason for us to not respect it
-            return float(retry_header)
-        except (TypeError, ValueError):
-            pass
-
-        # Last, try parsing `retry-after` as a date.
-        retry_date_tuple = email.utils.parsedate_tz(retry_header)
-        if retry_date_tuple is None:
-            return None
-
-        retry_date = email.utils.mktime_tz(retry_date_tuple)
-        return float(retry_date - time.time())
+        if "retry-after" in response_headers:
+            retry_header = response_headers["retry-after"]
+            try:
+                # note: the spec indicates that this should only ever be an integer
+                # but if someone sends a float there's no reason for us to not respect it
+                return float(retry_header)
+            except (TypeError, ValueError):
+                # Last, try parsing `retry-after` as a date.
+                retry_date_tuple = email.utils.parsedate_tz(retry_header)
+                if retry_date_tuple is not None:
+                    retry_date = email.utils.mktime_tz(retry_date_tuple)
+                    return float(retry_date - time.time())
+        return None
 
     def _calculate_retry_timeout(
         self,