Hello,
1) We don’t use XStore on 8theme.com. Another custom theme used here. So, 8theme.com is not related to your sie in any way. I’ll pass your request about our site speed to our developers. That’s strange because we don’t have the mentioned problem from the support staff. So, that would be great if you could send us video of the issue.
2) If you will use huge images and page size will be more than 3MB your site won’t be very fast. So, take a closer look at the content that you use on your site. Besides, you will need to install cache plugins once you finish developments, for example, Nitropack, WP Roket or W3 total cache.
Regards