Hello,
1) Page loads faster because I have optimized just a few images for the test. But not all of them! So, optimize all the images you use on your site.
2) We don’t recommend you to mix page builders on the site because it slows down the site. So, if you chose the Elementor as the main page builder then recreate the mentioned page using Elementor instead of the WPBakery.
3) I don’t see problem with the price filter or out of stock filter. Could you, please, provide us with video of the issue for better understanding?
Regards