Posts -> Add new: extremely slow

Hey,
I’ve updated to WP 4.2 on a website (actually, 4th update). While I didn’t have any issues in the previous environments, it’s a different story.

First, WordPress admin was asking to optimize the DB, but nothing would happen – I ended up installing the WP CLI on the server, and run the DB upgrade <- problem solved.

But, now, I’m facing something else – All the website is fast and working as usual, except the “add new” for posts and pages.
And I tried many things (yes I deactivated *all* the plugins, and cleared my cache), but it’s really slow.
Usually, every “click” takes less than 1 second (to list medias, to list posts, etc..) but when I click “All Posts -> Add new” it takes up to 60 seconds every single time. The only thing I haven’t tried is switching theme, because it’s a production website, so it’s not really possible for me to do that.

Is it something you experienced?