Slowness on the admin panel


We have recently upgraded to the latest Wordpress version (4.7.2) which became a nightmare, new post page stopped working, media library got extremely slow, although everything appeared to be fine on the front end. What I did was, I replaced every file with the latest working Wordpress version (4.6.1) and the new post page started working again but the slowness continues. There is a very annoying slowness on the backend, whenever there is a query to the mysql, like searching for an image, setting a featured image for a new post, approving comments, saving some new settings, it takes too long and at times, forever. I tried disabling all plugins, switching to a default theme, to no avail. We are on a pretty good VPS plan and the front end works fine so I know this is not a hosting issue either. So not a hosting issue, not a plugin/theme issue, possibly some left over PHP from the new version or the new MySql structure of the new version. I really don't know.

What I need is some Wordpress expert to look into my website and diagnose whatever it is causing this slowness, and hopefully fix it. I am willing to pay whatever it is the cost, just fix this for me. I don't need advice or some step by step, I need a solution fast. Do you have someone who can help me with this? Once again I will pay whatever it costs, I assume it shouldn't be more than a few hours or the very most a half day's job for someone who knows his way around Wordpress, but nevertheless please just help me out with it.

Thank you