diff --git a/backend/open_webui/retrieval/loaders/main.py b/backend/open_webui/retrieval/loaders/main.py
index f86f6fc2dc..218369fd93 100644
--- a/backend/open_webui/retrieval/loaders/main.py
+++ b/backend/open_webui/retrieval/loaders/main.py
@@ -303,13 +303,20 @@ class Loader:
         try:
             raw.decode('utf-8')
             return 'utf-8'
-        except UnicodeDecodeError:
-            pass
+        except UnicodeDecodeError as e:
+            first_non_utf8 = e.start
 
         # Use chardet as a hint, not as ground truth
         import chardet
 
-        detected = chardet.detect(raw)
+        # chardet is pure Python (~1.3s/MB), so sample around the first bad byte
+        window = 256 * 1024
+        sample_start = max(0, first_non_utf8 - window // 2)
+        sample = raw[sample_start : sample_start + window]
+        detected = chardet.detect(sample)
+        # A stray byte can sit far from the real payload, leaving the sample with nothing to read
+        if len(sample.translate(None, delete=bytes(range(128)))) < 64 and len(sample) < len(raw):
+            detected = chardet.detect(raw)
         detected_enc = (detected.get('encoding') or '').lower().replace('-', '').replace('_', '')
 
         # Map chardet's detected encoding to the correct superset codec.