reference to a different site in web error log

I am trying to troubleshoot the following entry that appears in my web server error log:

Slow ReadFile operation on file /home/nfs-clients/mod_pagespeed/v3/mysite.com/https,3A/,2Fmytestsite.com/tag/supplier-changes/feed/,: 103.867ms; configure SlowFileLatencyUs to change threshold\n

The issue for me is NOT the slow readfile remark. Instead, I am trying to understand the reference to mytestsite.com in a log for my production site. There is clearly something in my site that erroneously refers to a different site, but I cannot figure out what it is.

Similar entries in the log occur for many of the other post tags, as well as for some image files in uploads, in some plugin directories, and even for /wp-admin/images/browser.png

I have been through the database in all the wp_terms tables and found nothing. Same for all the entries in wp_options.

Can anyone help me figure out how these errors are being generated with a reference to another site? Thanks in advance.