diff --git a/electrum/gui/qt/custom_model.py b/electrum/gui/qt/custom_model.py
index 099e041b3..e9f9b05dc 100644
--- a/electrum/gui/qt/custom_model.py
+++ b/electrum/gui/qt/custom_model.py
@@ -62,12 +62,18 @@ class CustomModel(QtCore.QAbstractItemModel):
         parent.addChild(self, node)
 
     def index(self, row, column, _parent=None):
+        # Performance-critical function
+
         if not _parent or not _parent.isValid():
             parent = self._root
         else:
             parent = _parent.internalPointer()
 
-        if not QtCore.QAbstractItemModel.hasIndex(self, row, column, _parent):
+        # Open-coded
+        #   if not QtCore.QAbstractItemModel.hasIndex(self, row, column, _parent):
+        # the implementation is equivalent but it's in C++,
+        # so VM entries take up inordinate amounts of time (up to 25% of refresh()):
+        if row < 0 or column < 0 or row >= self.rowCount(_parent) or column >= self._columncount:
             return QtCore.QModelIndex()
 
         child = parent.child(row)