I have put the site online. I deleted the optimizations you gave us at https://generatepress.com/fastest-wordpress-theme/ to facilitate your visit (in addition it generates some issues on this site but is not the subject 🙂 )
There are :
– Header http://www.lesmatoirsdos.fr/
– Section http://www.lesmatoirsdos.fr/votre-projet-sur-mesure or http://www.lesmatoirsdos.fr/contact
As you can see, the effect isn’t the same in Header and Sections.