diff --git a/graphrag/config/environment_reader.py b/graphrag/config/environment_reader.py
index 258422666c..af1ed8dcca 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()
+    # Avoid isinstance for str first -- cheaper in general to check == type if type is frequently str
+    if type(value) is str:
+        return value.lower()
+    return value.value.lower()
 
 
 class EnvironmentReader:
@@ -41,14 +42,18 @@ def env(self):
     def _read_env(
         self, env_key: str | list[str], default_value: T, read: Callable[[str, T], T]
     ) -> T | None:
+        # Avoid creating a new list if already a list
         if isinstance(env_key, str):
-            env_key = [env_key]
+            env_keys = (env_key,)
+        else:
+            env_keys = env_key
 
-        for k in env_key:
+        for k in env_keys:
+            # Inline .upper() and avoid creating a variable for k.upper() unnecessarily.
+            # Call read once.
             result = read(k.upper(), default_value)
             if result is not default_value:
                 return result
-
         return default_value
 
     def envvar_prefix(self, prefix: KeyValue):
@@ -127,11 +132,13 @@ def float(
     ) -> float | None:
         """Read a float configuration value."""
         key = read_key(key)
-        if self.section and key in self.section:
-            return float(self.section[key])
-        return self._read_env(
-            env_key or key, default_value, lambda k, dv: self._env.float(k, dv)
-        )
+
+        section = getattr(self, "section", None)
+        if section is not None and key in section:
+            return float(section[key])
+        # Avoid lambda for hot path
+        env = self._env
+        return self._read_env(env_key or key, default_value, env.float)
 
     def list(
         self,