Editing Custom Menus Extremely Slow

I’m adding around 300+ sub pages to a custom menu. It’s already a monumental task. I’ve added them all and am not painstakingly dragging and dropping them in the correct places.

They’re moving VERY slow! Usually takes 10-20 seconds to move one page somewhere else or even to indent it! Moving a branch around takes almost a minute!

What is going on here?