Hi, I'm a 15 years experience fullstack developer and linux admin from Viet Nam (my skills and abilities are in my profile)
In your case: www.sp-batiment.com. I have some sugestions: I saw it already run on php7.2, right? so we will do opcode cache, object cache(need sourcecode modify). also, try to use php-fpm for better performance, turn on mysql (if used) cache, minify assets, decrease http requests, use cnd..
Also, I will make your pagespeed score rises up to at least 80, it's ten now, no good.
Thanks for your reading.
Khanhvu