Hi There,
There may be some script conflicting that is slowing down the Pro Editor loading time. If Pro is working without any issues, and just too slow, it can be related to memory. We can add the following lines in your wp-config.php file at the root of your installation:
define( 'WP_MEMORY_LIMIT', '256M' );
define( 'WP_MAX_MEMORY_LIMIT', '512M' );
Upon entering in this information everything should begin to work as expected unless your hosting provider has a limit on these parameters, which would keep them from taking effect. The next step would be to open up a support ticket with your hosting provider and ask them to increase your PHP memory limit to a higher setting, preferably 512M.