diff --git a/odoo/addons/base/models/res_users.py b/odoo/addons/base/models/res_users.py
index 2cd55dfbf25..5b691798ff3 100644
--- a/odoo/addons/base/models/res_users.py
+++ b/odoo/addons/base/models/res_users.py
@@ -1147,19 +1147,23 @@ class UsersImplied(models.Model):
         return super(UsersImplied, self).create(vals_list)
 
     def write(self, values):
+        if not values.get('groups_id'):
+            return super(UsersImplied, self).write(values)
         users_before = self.filtered(lambda u: u.has_group('base.group_user'))
         res = super(UsersImplied, self).write(values)
-        if values.get('groups_id'):
-            # add implied groups for all users
-            for user in self:
-                if not user.has_group('base.group_user') and user in users_before:
-                    # if we demoted a user, we strip him of all its previous privileges
-                    # (but we should not do it if we are simply adding a technical group to a portal user)
-                    vals = {'groups_id': [Command.clear()] + values['groups_id']}
-                    super(UsersImplied, user).write(vals)
-                gs = set(concat(g.trans_implied_ids for g in user.groups_id))
-                vals = {'groups_id': [Command.link(g.id) for g in gs]}
-                super(UsersImplied, user).write(vals)
+        demoted_users = users_before.filtered(lambda u: not u.has_group('base.group_user'))
+        if demoted_users:
+            # demoted users are restricted to the assigned groups only
+            vals = {'groups_id': [Command.clear()] + values['groups_id']}
+            super(UsersImplied, demoted_users).write(vals)
+        # add implied groups for all users (in batches)
+        users_batch = defaultdict(self.browse)
+        for user in self:
+            users_batch[user.groups_id] += user
+        for groups, users in users_batch.items():
+            gs = set(concat(g.trans_implied_ids for g in groups))
+            vals = {'groups_id': [Command.link(g.id) for g in gs]}
+            super(UsersImplied, users).write(vals)
         return res
 
 #