diff --git a/graphrag/config/environment_reader.py b/graphrag/config/environment_reader.py
index 258422666c..99b3e48ede 100644
--- a/graphrag/config/environment_reader.py
+++ b/graphrag/config/environment_reader.py
@@ -53,9 +53,13 @@ def _read_env(
 
     def envvar_prefix(self, prefix: KeyValue):
         """Set the environment variable prefix."""
-        prefix = read_key(prefix)
-        prefix = f"{prefix}_".upper()
-        return self._env.prefixed(prefix)
+        # Inline read_key logic to avoid function call overhead in hot path
+        if type(prefix) is str:
+            effective_prefix = prefix.lower()
+        else:
+            effective_prefix = prefix.value.lower()
+        effective_prefix = f"{effective_prefix}_".upper()
+        return self._env.prefixed(effective_prefix)
 
     def use(self, value: Any | None):
         """Create a context manager to push the value into the config_stack."""