The first thing you need to check is to be sure your server configuration meets the WordPress and the theme system requirements, check if there is enough memory for your site.
The other most common reason your WP Dashboard is slow is using a lot of 3-rd party plugins. Disable all plugins which are not required for the theme and check if the issue with slow admin panel still persists. If you see that your site backend loads fine then activate the plugins one by one to find which one/ones caused the problem. Also recheck your plugins settings, for example, if you are using W3 Total Cache then disable object cache.
Check if you don’t have errors with admin-ajax.php, as it may cause slowness not only of your site backend but fronted also. To sort out issues with admin-ajax.php you can use Heartbit Control plugin. Here you can find detailed instruction how to use it https://woorkup.com/diagnose-admin-ajax-php-causing-slow-load-times-wordpress/ .
If you are using WooCommerce check transients and delete expired ones as they also may be a problem. To manage it use the Transients Manager plugin.
Other one of the helpful measure is to optimize the database. Try to use WP Optimize plugin for this.
As a conclusion don’t forget to keep your WordPress core, the theme and plugins you are using up to date.
