[FIX] website_sale: very slow shop grid and product view

The problem is present because there are more than 1000 variants for each
product. For display, the price of each product is recalculated (instead
of looking only at the last product)

opw-766897

addons/website_sale/models/product.py

@@ -141,11 +141,13 @@ class ProductTemplate(models.Model):
     website_public_price = fields.Float('Website public price', compute='_website_price', digits=dp.get_precision('Product Price'))
 
     def _website_price(self):
-        self.mapped('product_variant_id')
-
-        for p in self:
-            p.website_price = p.product_variant_id.website_price
-            p.website_public_price = p.product_variant_id.website_public_price
+        # First filter out the ones that have no variant:
+        # This makes sure that every template below has a corresponding product in the zipped result.
+        self = self.filtered('product_variant_id')
+        # use mapped who returns a recordset with only itself to prefetch (and don't prefetch every product_variant_ids)
+        for template, product in zip(self, self.mapped('product_variant_id')):
+            template.website_price = product.website_price
+            template.website_public_price = product.website_public_price
 
     def _default_website_sequence(self):
         self._cr.execute("SELECT MIN(website_sequence) FROM %s" % self._table)