Some sites may benefit from Pagespeed which is disabled by default at minimum level with CoreFilters. 
(There is an internal cache that checks the source at regular intervals according to the frequency of the files: from 5 to 90 minutes...)
If you want to see changes immediately, or avoid 404 because of plugin / themes cache, leave it disabled.

To activate it, go into your cPanel, go at bottom, see at bottom plugin Nginx-Manager :

Choose your domain, and click 'Configure'

See section : 'Content optimizations' and 'Pagespeed', click on 'enabled', and click at bottom on button 'Save Settings'
(Wait 1 minute so that server will now take into account this modification)

There is also a stronger mode for it : Pass (PassThrough)...

It uses an internal cache system which check validity of cache every 5 minutes.
If it disturbs you because you update things a lot, and want to avoid some 404 due to cache of plugins/themes, you can disable it temporarily.

Go into your cPanel, go at bottom, see at bottom plugin Nginx-Manager :
See line 'Pagespeed_filters' = 'Pass', and click at bottom on button 'Save Settings'

(Wait 1 minute so that server will now take into account this modification)

Observe again your waterfall and speed:
https://tools.pingdom.com/

To deactivate pagespeed completely :
Click disabled for line 'Pagespeed' , and click at bottom on button 'Save Settings'

(Wait 1 minute so that server will now take into account this modification)