So I finish up another SharePoint 2010 -> 2016 migration and after themeing and everything is put in place, the next day my dev tells me he notices the sites slow. So I run a couple pages and sure enough each page takes 20-25 seconds to load.
Not sure what was going on since the test commands were 100% clean before hand and tested this a couple times in a test, followed all my documentation to a tee.
Checking the logs showed error messages of the SharePoint Managed account failing to access the User Profile Database. Checking the permissions set for the managed account in the database sure enough showed no access rights, when all other managed services accounts had basic connect rights.
Under security I found the security principal of the service account and under it’s User Mappings I added the DB_Access right for the managed service account. Sure enough this cleared the error message from the SharePoint logs and sites were loading super last. Always check your logs…