diff --git a/lektor_groupby/resolver.py b/lektor_groupby/resolver.py
index 67c218d..7391182 100644
--- a/lektor_groupby/resolver.py
+++ b/lektor_groupby/resolver.py
@@ -1,7 +1,5 @@
 from lektor.db import Page  # isinstance
-from typing import (
-    TYPE_CHECKING, NamedTuple, Dict, List, Set, Any, Optional, Iterable
-)
+from typing import TYPE_CHECKING, NamedTuple, Dict, List, Set, Any, Optional
 from .util import build_url
 from .vobj import VPATH, GroupBySource
 if TYPE_CHECKING:
@@ -74,7 +72,7 @@ class Resolver:
                 rv = subset.get(url)
                 if rv:
                     return GroupBySource(
-                        node, rv.key, rv.page).finalize(rv.config, rv.key_obj)
+                        node, rv.key, rv.config, rv.page).finalize(rv.key_obj)
         return None
 
     def resolve_virtual_path(self, node: 'SourceObject', pieces: List[str]) \
@@ -93,5 +91,5 @@ class Resolver:
             for rv in self._data.get(conf_key, {}).values():
                 if rv.equals(path, conf_key, vobj_key, page):
                     return GroupBySource(
-                        node, rv.key, rv.page).finalize(rv.config, rv.key_obj)
+                        node, rv.key, rv.config, rv.page).finalize(rv.key_obj)
         return None
diff --git a/lektor_groupby/vobj.py b/lektor_groupby/vobj.py
index 33fd441..c09c49a 100644
--- a/lektor_groupby/vobj.py
+++ b/lektor_groupby/vobj.py
@@ -34,6 +34,7 @@ class GroupBySource(VirtualSourceObject):
         self,
         record: 'Record',
         key: str,
+        config: 'Config',
         page_num: Optional[int] = None
     ) -> None:
         super().__init__(record)
@@ -41,6 +42,7 @@ class GroupBySource(VirtualSourceObject):
         self.__key_obj_map = []  # type: List[Any]
         self._expr_fields = {}  # type: Dict[str, Expression]
         self.key = key
+        self.config = config
         self.page_num = page_num
 
     def append_child(self, child: 'Record', key_obj: Any) -> None:
@@ -68,28 +70,31 @@ class GroupBySource(VirtualSourceObject):
     #   Evaluate Extra Fields
     # -------------------------
 
-    def finalize(self, config: 'Config', key_obj: Optional[Any] = None) \
+    def finalize(self, key_obj: Optional[Any] = None) \
             -> 'GroupBySource':
-        self.config = config
         # make a sorted children query
         self._query = FixedRecordsQuery(self.pad, self.__children, self.alt)
-        self._query._order_by = config.order_by
+        self._query._order_by = self.config.order_by
         del self.__children
         # set indexed original value (can be: str, int, float, bool, obj)
         self.key_obj = key_obj or most_used_key(self.__key_obj_map)
         del self.__key_obj_map
-        # evaluate slug Expression
-        self.slug = config.eval_slug(self.key, on=self)
-        if self.slug and self.slug.endswith('/index.html'):
-            self.slug = self.slug[:-10]
 
         if key_obj:  # exit early if initialized through resolver
             return self
         # extra fields
-        for attr in config.fields:
-            self._update_attr(attr, config.eval_field(attr, on=self))
+        for attr in self.config.fields:
+            self._update_attr(attr, self.config.eval_field(attr, on=self))
         return self
 
+    @cached_property
+    def slug(self) -> Optional[str]:
+        # evaluate slug Expression once we need it
+        slug = self.config.eval_slug(self.key, on=self)
+        if slug and slug.endswith('/index.html'):
+            slug = slug[:-10]
+        return slug
+
     # -----------------------
     #   Pagination handling
     # -----------------------
diff --git a/lektor_groupby/watcher.py b/lektor_groupby/watcher.py
index 5982a23..136e39c 100644
--- a/lektor_groupby/watcher.py
+++ b/lektor_groupby/watcher.py
@@ -117,12 +117,12 @@ class Watcher:
         # update internal object storage
         alt = args.record.alt
         if slug not in self._state[alt]:
-            src = GroupBySource(self._root_record[alt], slug)
+            src = GroupBySource(self._root_record[alt], slug, self.config)
             self._state[alt][slug] = src
         else:
             src = self._state[alt][slug]
 
-        src.append_child(args.record, obj)  # obj is used as "group" string
+        src.append_child(args.record, obj)
         # reverse reference
         VGroups.of(args.record).add(args.key, src)
         return slug
@@ -137,7 +137,7 @@ class Watcher:
         del self._rmmbr
         for vobj_list in self._state.values():
             for vobj in vobj_list.values():
-                yield vobj.finalize(self.config)
+                yield vobj.finalize()
         # cleanup. remove this code if you'd like to iter twice
         del self._model_reader
         del self._root_record