The following error log appears in
/var/log/apache2/error.log in Debian/Ubuntu or in
/var/log/httpd/error_log on CentOS/Redhat operating systems:
[Tue Oct 03 10:32:41.965156 2017] [pagespeed:error] [pid 23476] [mod_pagespeed 188.8.131.52-0 @23476] Slow ReadFile operation on file /var/cache/mod_pagespeed/v3/example.com/https,3A/,2Fwww.example.com/forums,: 4039.37ms; configure SlowFileLatencyUs to change threshold
Slow disk I/O performance.
Increase the disk performance, for example by increasing disk I/O limits or switching server's disks to faster ones.