Hi,
I'm Prestashop Expert for over 6 years experience,
I have worked before on similar projects.
I use google page speed inseight to see the slowness causes.
I can be from images size, css and js compression, server reponse time, synchrone vs asynchrone call to js and css files, using browser cache expiration date, using mysql or file system cache.
I can use custom tools, like plugin apache pagespeed and others.
Feel free to contact me for more details.
Thanks.