WordPress backend wp-admin speed issue

Hi, we have a WP website that uses a bunch of plugins. The site (front end) runs perfect and is fast (pagespeed 86%). But the backend is dead slow. Adding a post or something takes sometimes 15-20s. We have other sites running on the same server (we are the server owner) that are working perfectly and very responsive. I assume a problem or conflict with a plugin or something. We are looking for someone that will completely analyze the problem and provide a solution without loss of functionality. So we don't want someone that says 'get a faster server' or 'disable your plugins', but someone that gets to the bottom of the problem. The goal is to add a new page or post in 2-5 seconds. We have already created a staging version of the site where you will be doing your analysis. After your report we should be able to do the required changes on the live site on our own.