diff --git a/litellm/proxy/proxy_server.py b/litellm/proxy/proxy_server.py
index 48025863641..bd5b5309e0f 100644
--- a/litellm/proxy/proxy_server.py
+++ b/litellm/proxy/proxy_server.py
@@ -5302,13 +5302,15 @@ async def async_data_generator(
 ):
     verbose_proxy_logger.debug("inside generator")
     try:
-        # Use a list to accumulate response segments to avoid O(n^2) string concatenation
-        str_so_far_parts: list[str] = []
         error_message: Optional[str] = None
         requested_model_from_client = _get_client_requested_model_for_streaming(
             request_data=request_data
         )
         model_mismatch_logged = False
+        # Use a running string instead of list + join to avoid O(n^2) overhead.
+        # Previously "".join(str_so_far_parts) was called every chunk, re-joining
+        # the entire accumulated response. String += is O(n) amortized total.
+        _str_so_far: str = ""
         async for chunk in proxy_logging_obj.async_post_call_streaming_iterator_hook(
             user_api_key_dict=user_api_key_dict,
             response=response,
@@ -5319,12 +5321,12 @@ async def async_data_generator(
                 user_api_key_dict=user_api_key_dict,
                 response=chunk,
                 data=request_data,
-                str_so_far="".join(str_so_far_parts),
+                str_so_far=_str_so_far if _str_so_far else None,
             )
 
             if isinstance(chunk, (ModelResponse, ModelResponseStream)):
                 response_str = litellm.get_response_string(response_obj=chunk)
-                str_so_far_parts.append(response_str)
+                _str_so_far += response_str
 
             chunk, model_mismatch_logged = _restamp_streaming_chunk_model(
                 chunk=chunk,