WP Admin extremely slow - wordpress

Question

The WP back end of a site I'm working on (It's a multisite) takes about 25 seconds to load.
Everything was working fine until yesterday and the front end still works perfectly well. All other sites on the same server run just as well, so it MUST be a WP back end issue.
I don't remember exactly what change it was that made it so slow. I remember updating WP recently (to version 3.4.2), adding some plugins on one of the sites and changing the max upload file size.
I tried to disable all the plugins, changing the themes back to default, changing the max file size back, and adding define('WP_MEMORY_LIMIT', '1024M'); (and other values) to WP-config but none of it helped.
Also tried to 'Update network', but I got an error - couldn't connect to host.
Any ideas?

Answer 1

I got in touch with our network admin and we resolved the issue.
I will copy his answer here. Hope it helps someone.
Does Wordpress use 'self-referential URLs' ? What I mean by this is...
is wordpress trying to access it's own templates/css using fully
qualified domain names in the URL (e.g. http://example.co.uk/someurl )
Because we use Network Address Translation (NAT) on our firewalls to
hide the real IP address of the server, it has the side effect that if
the server tries to access it's own URLs, it will try to send the
traffic to the external interface on our firewall, which is where the
DNS resolves to.
The fix for this is very simple - we just add the site url into the
/etc/hosts file so that the server knows to use it's own IP address
instead of the address on the firewall.
So he added our address to the hosts file and now it works perfectly.
Awesome.

Answer 2

I've seen this before where the admin pages are trying to poll external Wordpress sites for details of Wordpress upgrades, plugin updates and Wordpress news. If there's no proper access (because of firewall restrictions, bad DNS, etc) then the page has to wait for the HTTP requests (I think WP uses cURL) to timeout.
If you're still unable to identify the cause I'd recommend a catch-all solution of installing xdebug and profiling the page with webgrind, xcachegrind, etc

Answer 3

Had the same problem for a week and now the problem of very slow WP-admin was solved!
Before, I cannot access my sites if I use incognito or I am not logged in as WP user, but all times in the wp-admin, it takes me 40 seconds- minute or even never.
Solution that worked:
I accessed the files in the file manager using the CPanel, and I saw so many unused and unnecessary folders and themes and that's the reason that causes the very slow access to admin.
It was because during the days of being a newbie, I stuffed a lot of files in the Public Http and that made it congested.
I logged in to another CPanel account that I bought personally before, and compared the folders of the "proper" versus the "congested" and compress, backed-up and deleted all the unnecessary.
My host: Hostgator, responded well also.
Hope this would help others.

Answer 4

I also had a very slow Dashboad in wordpress. Reading the James C´s answer, I realized that my site is located in a corporate intranet behind a firewall to access internet.
James C answered:
"I've seen this before where the admin pages are trying to poll external Wordpress sites for details of Wordpress upgrades, plugin updates and Wordpress news. If there's no proper access (because of firewall restrictions, bad DNS, etc) then the page has to wait for the HTTP requests (I think WP uses cURL) to timeout."
My solution was avoid all the internet conections: (1) disable all the wordpress updates using the wordpress plugin "Disable all wordpress updates". (2) activate de wordpress pluging "Disable google fonts"
After these two plugin activations, the Dashboard works to a suitable speed.