Over the past few months the admin pages on my wordpress multisite site (around 80 sites) have been taking around 20 seconds to load.  I am running the latest version of wordpress (version 4.1).  

When I renamed the plugins folder to "plugins-deactivated" the admin area returned to a usual peppy speed of three seconds.  However, when I renamed the folder back to "plugins" the site returned to it's usual 20 load times.  I then deactivated all of the plugins and the admin area remained at it's usual slow speed, even though the plugins were deactivated.

I deleted all of the plugins and re-uploaded them one by one - the admin slowed with each additional plugin, no matter which plugin I uploaded.  The obvious conclusion to draw is that Wordpress is loading each and every single plugin whether activated or not.

My question - how can I validate this assumption, and also, how can I further diagnose the problem?

Some other facts

 1. The site is a wordpress multisite with approx 80 sites
 2. It is running on Windows Azure
 3. The problem started in the past few months
 4. All plugins are activated
 5. I am running the query monitor plugin
 6. The installation is about eight years old

How can I find where the slowness originates?

***Update 1:*** I have used the Chrome Dev Tools - and about 95% of the load speed is in the initial Get request - which would seem to suggest that the problem is happening on the server, not any of the ancillary load items

***Update 2:*** The mere presence of the Gravity Forms plugin seems to add about 9 seconds to the load time of the site.  It is happening when the plugin is deactivated.