Manual:$wgUseFileCache

Details
Set to   to activate server-side file caching.

This will cache static pages for non-logged-in users to reduce database traffic on public sites.

If  is set to  ,   and   will automatically be set to  .

(Part of that automatically done after r46374.)

Specify where you want the cache files to go in your server filesystem using  .

You must create the directory yourself though.