diff --git a/addons/crm/models/crm_lead.py b/addons/crm/models/crm_lead.py
index 9d4244b58b7..8a8b618edf6 100644
--- a/addons/crm/models/crm_lead.py
+++ b/addons/crm/models/crm_lead.py
@@ -1246,11 +1246,19 @@ class Lead(models.Model):
         :param int team_id: salesteam to assign
         """
         update_vals = {'team_id': team_id} if team_id else {}
-        for index, lead in enumerate(self):
-            if user_ids:
-                update_vals['user_id'] = user_ids[index % len(user_ids)]
-            if update_vals:
-                lead.write(update_vals)
+        if not user_ids:
+            self.write(update_vals)
+        else:
+            lead_ids = self.ids
+            steps = len(user_ids)
+            # pass 1 : lead_ids[0:6:3] = [L1,L4]
+            # pass 2 : lead_ids[1:6:3] = [L2,L5]
+            # pass 3 : lead_ids[2:6:3] = [L3,L6]
+            # ...
+            for idx in range(0, steps):
+                subset_ids = lead_ids[idx:len(lead_ids):steps]
+                update_vals['user_id'] = user_ids[idx]
+                self.env['crm.lead'].browse(subset_ids).write(update_vals)
 
     # ------------------------------------------------------------
     # TOOLS
diff --git a/addons/crm/tests/test_crm_lead_convert_mass.py b/addons/crm/tests/test_crm_lead_convert_mass.py
index 05c347301a6..d24de28b39e 100644
--- a/addons/crm/tests/test_crm_lead_convert_mass.py
+++ b/addons/crm/tests/test_crm_lead_convert_mass.py
@@ -24,7 +24,7 @@ class TestLeadConvertMass(crm_common.TestLeadConvertMassCommon):
         with self.assertQueryCount(user_sales_manager=0):
             test_leads = self.env['crm.lead'].browse(test_leads.ids)
 
-        with self.assertQueryCount(user_sales_manager=330):
+        with self.assertQueryCount(user_sales_manager=252):
             test_leads.handle_salesmen_assignment(user_ids=user_ids, team_id=False)
 
         self.assertEqual(test_leads.team_id, self.sales_team_convert | self.sales_team_1)
@@ -42,7 +42,7 @@ class TestLeadConvertMass(crm_common.TestLeadConvertMassCommon):
         with self.assertQueryCount(user_sales_manager=0):
             test_leads = self.env['crm.lead'].browse(test_leads.ids)
 
-        with self.assertQueryCount(user_sales_manager=342):
+        with self.assertQueryCount(user_sales_manager=217):
             test_leads.handle_salesmen_assignment(user_ids=user_ids, team_id=team_id)
 
         self.assertEqual(test_leads.team_id, self.sales_team_convert)
@@ -166,7 +166,7 @@ class TestLeadConvertMass(crm_common.TestLeadConvertMassCommon):
         test_leads = self._create_leads_batch(count=50, user_ids=[False])
         user_ids = self.assign_users.ids
 
-        with self.assertQueryCount(user_sales_manager=1472):
+        with self.assertQueryCount(user_sales_manager=1347):
             mass_convert = self.env['crm.lead2opportunity.partner.mass'].with_context({
                 'active_model': 'crm.lead',
                 'active_ids': test_leads.ids,