Install Celeris, activate your license, configure the cache and read the measurements.
Open Recommendations: it lists what is not enabled yet, in order of impact, with an Enable button on each card. Then Measurements: compare with and without.
When PHP runs under nginx, paste the block from Celeris › Server into your virtual host (Plesk: additional nginx directives). Until then the cache is served by PHP, which already skips WordPress.