admin-ajax.php slows down, but cant figure out which plugin. Tried to follow these steps: https://woorkup.com/diagnose-admin-ajax-php-causing-slow-load-times-wordpress/
An option is to manually disable all plugins and enable them one-by-one and see when trouble occurse again.
Another option is to use a WordPress profiler. I would highly recommend using InterconnectIT's WordPress Performance Profiler. This plugin will profile all or a certain amount of requests and show you what plugins take a long time to load.