Back-office delays can have a different cause from slow public pages. An anonymous HTML page cache should not cache administrative screens or actions.
Measure the affected task
Record whether login, order lists, product editing, search or a module configuration is slow. Compare quiet and busy periods. In a test copy, inspect PHP logs, slow SQL queries, scheduled jobs and modules loading data or scripts in the admin.
Change one thing at a time
Check memory, CPU, a supported PHP version and OPcache before investigating expensive queries. Update modules in a controlled way; do not disable them blindly in production. Back up first and measure again after each correction. If only public pages are slow, investigate the front office instead.
Read the guide to faster public pages
Source : PrestaShop Help Center.
Public caching does not solve every slow back office
Administration uses private, dynamic data that must not be served as public pages. Identify the slow screen and check response times, modules, external calls and queries before changing Smarty, PHP or the database. A faster catalogue does not demonstrate a faster orders screen.