Slow Admin sections after 4.3 update

I am just baffled so hopefully someone knows about this:

I updated the WP to 4.3 on my multisites and ever since, the admin sections of each site has been slow – I mean 4 minutes to load in a page slow.

The frontend of the sites are fine – just as speedy as ever. Just going from Plugins to Pages within the admin areas takes forever.

I have removed plugins, checked system status and tried different themes (2015). No speed increase.

What makes things odder – I can add a new folder on my server and load in WP 4.3 independently (basically creating a new MS using the same files) and using the same plugins, database and themes as my main multisite, I have no lag time in switching admin areas.

I could rebuild each module but there are 26 of them and each has WooCommerce and different products.

Any clues out there?