Wordpress admin extremely slow

I helped a client build a site on wordpress 3.3 and recently the client updated to wordpress 4.3 The front end has no issues and is very snappy. However it takes over a minute to log into the admin side. And each page click on the admin side is very slow.

The client site is currently live so it's hard to disable all plugins and test one by one. Is there a way to check what is causing the slow down without disabling the site?