diff --git a/graphrag/config/environment_reader.py b/graphrag/config/environment_reader.py
index 258422666c..08f31b2d05 100644
--- a/graphrag/config/environment_reader.py
+++ b/graphrag/config/environment_reader.py
@@ -18,9 +18,10 @@
 
 def read_key(value: KeyValue) -> str:
     """Read a key value."""
-    if not isinstance(value, str):
-        return value.value.lower()
-    return value.lower()
+    # Use type(value) is str for faster type checking than isinstance
+    if type(value) is str:
+        return value.lower()
+    return value.value.lower()
 
 
 class EnvironmentReader: