diff --git a/baseten/sandbox/_common.py b/baseten/sandbox/_common.py
index fba8107..c95c228 100644
--- a/baseten/sandbox/_common.py
+++ b/baseten/sandbox/_common.py
@@ -437,9 +437,12 @@ def response_lines(response: httpx.Response) -> Generator[str, None, None]:
     decoder = codecs.getincrementaldecoder("utf-8")(errors="replace")
     buffer = ""
     for chunk in response.iter_bytes():
-        buffer += decoder.decode(chunk)
-        *lines, buffer = _LINE_BREAK.split(buffer)
-        yield from lines
+        text = decoder.decode(chunk)
+        buffer += text
+        # Split only when a line ends, so a long line isn't rescanned per chunk.
+        if "\n" in text:
+            *lines, buffer = _LINE_BREAK.split(buffer)
+            yield from lines
     buffer += decoder.decode(b"", final=True)
     if buffer:
         yield buffer
@@ -450,10 +453,12 @@ async def response_lines_async(response: httpx.Response) -> AsyncGenerator[str,
     decoder = codecs.getincrementaldecoder("utf-8")(errors="replace")
     buffer = ""
     async for chunk in response.aiter_bytes():
-        buffer += decoder.decode(chunk)
-        *lines, buffer = _LINE_BREAK.split(buffer)
-        for line in lines:
-            yield line
+        text = decoder.decode(chunk)
+        buffer += text
+        if "\n" in text:
+            *lines, buffer = _LINE_BREAK.split(buffer)
+            for line in lines:
+                yield line
     buffer += decoder.decode(b"", final=True)
     if buffer:
         yield buffer
diff --git a/tests/sandbox/test_common.py b/tests/sandbox/test_common.py
index b8e4949..cb9f460 100644
--- a/tests/sandbox/test_common.py
+++ b/tests/sandbox/test_common.py
@@ -146,6 +146,8 @@ async def aclose(self) -> None:
         # A character split across chunks, and invalid UTF-8 replaced.
         (("é\n".encode()[:1], "é\n".encode()[1:]), ["é"]),
         ((b"a\xffb\n",), ["a�b"]),
+        # A long line across many chunks, ended by \r\n split across two.
+        ((*[b"x" * 1024] * 64, b"\r", b"\ny"), ["x" * 65536, "y"]),
     ],
 )
 class TestResponseLines: