diff --git a/odoo/fields.py b/odoo/fields.py
index adeda5176b3..6ef211e12a8 100644
--- a/odoo/fields.py
+++ b/odoo/fields.py
@@ -58,14 +58,13 @@ def resolve_mro(model, name, predicate):
         classes are ignored.
     """
     result = []
-    for cls in type(model).mro():
-        if not is_registry_class(cls):
-            value = cls.__dict__.get(name, Default)
-            if value is Default:
-                continue
-            if not predicate(value):
-                break
-            result.append(value)
+    for cls in model._model_classes:
+        value = cls.__dict__.get(name, Default)
+        if value is Default:
+            continue
+        if not predicate(value):
+            break
+        result.append(value)
     return result
 
 
@@ -4100,6 +4099,6 @@ def apply_required(model, field_name):
 # pylint: disable=wrong-import-position
 from .exceptions import AccessError, MissingError, UserError
 from .models import (
-    check_pg_name, expand_ids, is_definition_class, is_registry_class,
+    check_pg_name, expand_ids, is_definition_class,
     BaseModel, IdType, NewId, PREFETCH_MAX,
 )
diff --git a/odoo/models.py b/odoo/models.py
index e4a954fdd26..29bfea68f5c 100644
--- a/odoo/models.py
+++ b/odoo/models.py
@@ -2986,6 +2986,12 @@ class BaseModel(metaclass=MetaModel):
         if cls._setup_done:
             return
 
+        # the classes that define this model, i.e., the ones that are not
+        # registry classes; the purpose of this attribute is to behave as a
+        # cache of [c for c in cls.mro() if not is_registry_class(c))], which
+        # is heavily used in function fields.resolve_mro()
+        cls._model_classes = tuple(c for c in cls.mro() if getattr(c, 'pool', None) is None)
+
         # 1. determine the proper fields of the model: the fields defined on the
         # class and magic fields, not the inherited or custom ones
 
@@ -2997,8 +3003,9 @@ class BaseModel(metaclass=MetaModel):
 
         # collect the definitions of each field (base definition + overrides)
         definitions = defaultdict(list)
-        for klass in reversed(cls.mro()):
-            if is_definition_class(klass):
+        for klass in reversed(cls._model_classes):
+            # this condition is an optimization of is_definition_class(klass)
+            if isinstance(klass, MetaModel):
                 for field in klass._field_definitions:
                     definitions[field.name].append(field)
         for name, fields_ in definitions.items():