Sometime you dont know why your prestashop site run very slow, may be this tip will help you. When you use prestashop default version it has more unused modules/blocks but source of them (javascrip/css) still loaded on frontend, because unused modules assigned to header Hook.
To remove those source please do some steps below : 1 - Login to Backend
2 - Go to Modules and Services > Positions 3 - Go to Header box 4 - UnHook modules unused.
5 - ctr + F5 to reload frontend site. Tip : Facebook like box block is one of unused modules ussually assigned to hook and source of it make site run slow. Hope This article helpful for you.