--- a/configmix/config.py	Sun Jan 02 20:43:24 2022 +0100
+++ b/configmix/config.py	Mon Jan 03 00:11:41 2022 +0100
@@ -32,13 +32,17 @@
 from .constants import REF_NAMESPACE, NONE_FILTER, EMPTY_FILTER
 try:
     from ._speedups import (fast_unquote, fast_quote, fast_pathstr2path,
-                            _fast_split_ns, _fast_split_filters)
+                            _fast_split_ns, _fast_split_filters,
+                            _fast_interpolate_variables,
+                            _sync_MISSING)
 except ImportError:
     fast_unquote = None
     fast_quote = None
     fast_pathstr2path = None
     _fast_split_ns = None
     _fast_split_filters = None
+    _fast_interpolate_variables = None
+    _sync_MISSING = None
 
 
 _MARKER = object()
@@ -873,7 +877,7 @@
         else:
             return obj
 
-    def interpolate_variables(self, s):
+    def py_interpolate_variables(self, s):
         """Expand all variables in the single string `s`"""
         len_s = len(s)
         if len_s < 4:
@@ -943,6 +947,18 @@
             self.__interpolation_cache[s] = res
         return res
 
+    if _fast_interpolate_variables:
+
+        def fast_interpolate_variables(self, s):
+            return _fast_interpolate_variables(
+                self, s, self.__interpolation_cache)
+
+        interpolate_variables = fast_interpolate_variables
+
+    else:
+
+        interpolate_variables = py_interpolate_variables
+
     def _apply_filters(self, filters, value):
         for name in filters:
             try:
@@ -1221,3 +1237,7 @@
     def __repr__(self):
         r = "_JailedConfiguration(rootpath=%s)" % n(repr(self._path))
         return r
+
+
+if _sync_MISSING:
+    _sync_MISSING(_MISSING)