Good find. Some recent updates were made to improve performance for large menus, and these will also help with performance: https://core.trac.wordpress.org/ticket/30738 and https://core.trac.wordpress.org/ticket/30741
You might also like:
https://make.wordpress.org/flow/2015/06/14/menus-in-the-admin-and-the-customizer-ux-flow-performance-comparisons/
I love the idea of managing page order right from the pages section (which I think is what the Nest Pages plugin does if I’m understanding the description correctly) and allowing that order to be used as one possible menu choice. There is still a need to manage menus outside that space too though, and a dedicated menu management area covers that.
After the latest nightly WordPress update (4.3-alpha-32949) I’m getting an unresponsive script error now when trying to use the Customizer Menu with my large menu system. Using Firefox 38.0.5 on Mac OS 10.9.5. The debugger is pointing at line 8 of the load-scripts.php file at some javascript – possibly related to the underscores.js library.
Also when it was working (prior to the unresponsive script issue after the latest nightly update) there was some definite UI craziness when trying to nest/un-nest menu items using the angle bracket buttons. Sometimes things worked great. Other times menu items wouldn’t budge and with no explanation for why the one worked and the other didn’t. And then sometimes it would totally get lost and nothing would happen and you’d have to save and hope you got what you wanted. Or back out and try again. Anyway there wasn’t enough “communication” between user choices and the Customizer Menu. It made for a poor user experience.
Thank you for the early testing on this!
32769 was closed as resolved, so I am marking this thread as resolved.