Let's see who's the expert of all WordPress experts for all future viewers !.
I just migrated a website from one domain to another domain for testing purposes yeah?
The new domain name uses the same registrar & redirects to the same server ip as the original domain yeah?
The installation on the new domain is an EXACT CLONE of the original, cloned using Cpanel cloner within the same cpanel, then used better search and replace to change all the missed domain entries in the database to the new domain (1020 in total were found)
Cleared the cache (wp rocket)
The website has only a few (8 or so) plugins installed.
The original domain is faster than the new domain installation by 500ms to 1000ms depending on the page. The difference in speed and responsiveness is visibly noticeable and it's mainly a difference in TTFB.
I saw this happen in the past using other cloning means such as backup plugins.
If your brain has enough grey matter to even comprehend how this is possible, i can't wait to see your response !