You have a lot of problems, just looking at your home page:
You are loading 36 javascripts! (1.5mb)
- they are not all needed on the home page
- only load the ones you need for that page
- combine js into fewer files to reduce http requests
You are loading 14 CSS files! (0.99mb)
- again they probably aren’t all needed
- your would be better off reducing the number of CSS files to 1 for your main site, and 1 for Google fonts
- I’m wondering why http://www.walt-interiors.be/wp-content/plugins/js_composer/assets/css/js_composer.css?ver=4.5.2 is being called, isn’t that some sort of page builder for the back end?