Having to deal with making changes to my menu via Appearance>Menu has been a nightmare ever since WordPress 5.8.
It will freeze and a dialogue will pop up saying this page is not responding or moving menu items will take forever.
I looked up this issue and I found the answer - https://wordpress.com/forums/topic/does-your-menu-editor-freeze-or-slow-got-a-fix/.
- I changed my theme and the menu worked smoothly just as it did before the 5.8 update.
- I deactivated every plugin with the Pro Child theme active and the menu issues did not change.
I think this is a bug in relation to the Pro theme and WordPress version 5.8.*.
Thanks and regards,