All your nominated DNS servers are actually active and reachable.
Your other thread indicates that you are isolated from the internet, which is important information. In that case, your DNS servers must be configured to give definitive no such domain responses, rather than trying to forward to, or redirect to, the root servers.
It must resolve them as non-existent. If it simply doesn’t respond, or refers to the root server, you will get long timeouts.
Basically, if you have an isolated island, and there is any possibility of a lookup for a name outside that island, and you are not prepared to wait for a timeout, you need to make yourself the authority for the root domain and all intermediates, to your private domain, and to your part of in_addr.arpa.
I can verify this. I will a lot of time setup a FreePBX at home and then go oonsite and install the server. I will still have the old IP info in it from my home and my internal DNS at home. I will load the Admin UI REALL SLOW until I get in there and update the IP / DNS Info. FreePBX I figure is using the DNS to do all kinds of things.
I am doubtful that any of us community members are going to spend the time debugging our freepbx instances to give you exactly what DNS names the system is looking up so you can apply them to your niche install.
Run a tcpdump on the server while you are loading the admin page, and study the pcap to see what DNS lookups the server is doing and make sure those actually resolve.
I don’t know the specific domain names it is trying to resolve. I just put in the correct dns server for the network that can resolve the domain names. FreePBX is checking activation info, license key info… All kinds of stuff. If it cannot resolve them it is going to timeout.
I would to a TCP dump and look at all the records it is trying to find.
I tested the theory that the slowness is caused by the DNS, by resolding the names on your list. They all errored in under 1ms. Then I drilled a hole in the firewall, from the FreePBX box to our DNS server that we use for the LAN. It can also resolve all of these names, in under 1 ms, but the 3x slow pages are still as slow as they used to be, so DNS is not the culprit.