We have a multisite installation where the backend only is affected but only when domain mapping is set up.
The base domain .com is fine. Sites that are subfolders (.com/es) are fine. Sites that are on their own site using domain mapping (.co.uk) are slow so we tested it using a new domain that is on the same registrar as the slow domain with the same DNS records. A blank site hosted on the test domain is fine. A copy of the slow site on a subfolder is fine but a copy of the slow site on a test domain is slow too.Everything points to TTFB.
Here's a test we did:
- Copy of domain.co.uk to domain.com/testsite/ > Fast
- Blank site on the domain.com multisite on otherdomain.com > Fast
- Copy of domain.co.uk to otherdomain.com > Slow
We thought it was something to do with nameservers but it isn't. Nothing on our WP setup would point to anything slow.