First check to see if your website is using Mod_Pagespeed with this website: https://ismodpagespeedworking.com/
Alternatively, run:
curl -D- https://www.domain.com/ | less
If you do see the header, but it doesn't look like PageSpeed is making any changes to your page, it is possible that none of the active filters are finding anything to rewrite.
Try comparing your page with PageSpeed off and with the collapse_whitespace filter enabled:
curl -D- 'https://example.com?ModPagespeed=off' | less curl -D- 'https://example.com?ModPagespeed=on&ModPagespeedFilters=collapse_whitespace' | less
If you experience issues with your website or need to see everything as it is by default or notice that the changes you are making on your website are not reflecting in real-time), turn Mod_PageSpeed "Off".
https://www.domain.com?ModPagespeed=off
Turning the module on and off
To turn PageSpeed temporarily off, just set this on your .htaccess using examples found at this Dropbox URL:
ModPagespeed standby OR ModPagespeed off
To turn it on again, replace the "standby" or "off" with "on"
If you are a VPS or dedicated customer with system admin access to top-level Apache configuration and want to turn it off completely, use:
ModPagespeed unplugged
You can disable it via EasyApache 4 if you are on cPanel & WHM server.