Regarding your Appearance > Menus, I’ve tested loading it 7 times and it loaded successfully for 4 times. it returned a fatal error for the rest of the time or specifically 3 times. During the times that it loaded, the menu page was very slow. This means that your server is struggling to handle a lot of menus even Ruenel increased the
Regretfully, we can do nothing for your server’s limitation. Believe it or not, this one is an issue with your web host. But, it’s not really recommended to have lots of menu items in WordPress in general. Quality web hosts can handle many menus though.
Regarding the UberMenu style configurations not working in your site, it could be related to your web host or WordPress setup as the said options work fine in my test site. I couldn’t do testing in your live site as it will interrupt your site’s operation though so I can’t pin point the exact cause.
To test if your server is the only thing causing the issue, setup a staging site with fresh WordPress install and then only install our X and UberMenu in it and then add in menu items one by one until you experience lagging in the menu page. This will tell you how many menu items your server is able to handle.
If there’s no issue with the server test, add in the third party plugins you’re using in your live site to the staging site one by one. This will tell you if a third party plugin is causing the issue.
Thank you for your cooperation.