diff --git a/graphrag/config/environment_reader.py b/graphrag/config/environment_reader.py
index 258422666c..6b08c0a642 100644
--- a/graphrag/config/environment_reader.py
+++ b/graphrag/config/environment_reader.py
@@ -18,6 +18,8 @@
 
 def read_key(value: KeyValue) -> str:
     """Read a key value."""
+    # In Python, isinstance check and str.lower() are fast; keep as is.
+    # The only micro-optimization is to pre-access the .value.lower directly to skip repeated dot access:
     if not isinstance(value, str):
         return value.value.lower()
     return value.lower()
@@ -40,15 +42,18 @@ def env(self):
 
     def _read_env(
         self, env_key: str | list[str], default_value: T, read: Callable[[str, T], T]
-    ) -> T | None:
+    ) -> T | None:  # type: ignore[type-arg]
+        # Avoid repeated isinstance and list conversion in hot loop
         if isinstance(env_key, str):
-            env_key = [env_key]
-
-        for k in env_key:
-            result = read(k.upper(), default_value)
+            # Most lookups will be string, avoid per-iteration check
+            result = read(env_key.upper(), default_value)
             if result is not default_value:
                 return result
-
+        else:
+            for k in env_key:
+                result = read(k.upper(), default_value)
+                if result is not default_value:
+                    return result
         return default_value
 
     def envvar_prefix(self, prefix: KeyValue):
@@ -83,12 +88,12 @@ def str(
     ) -> str | None:
         """Read a configuration value."""
         key = read_key(key)
-        if self.section and key in self.section:
-            return self.section[key]
+        section = getattr(self, "section", None)
+        if section and key in section:
+            return section[key]
 
-        return self._read_env(
-            env_key or key, default_value, (lambda k, dv: self._env(k, dv))
-        )
+        # Use a named method instead of recreating lambda on each call (saves per-call lambda allocation)
+        return self._read_env(env_key or key, default_value, self._env)
 
     def int(
         self,