This question already has an answer here:
I do not think that there is any sane way to sort this issue with the amount of posts you have except than setting
true has huge impact on performance and aren’t really meant for use in custom post types.
If you do not need a parent/child relationship between posts like the build-in post type
page have, then you should really set
hierarchical to `false. If you really need that relationship, then you are pretty much stuffed unfortunately. With the amount of posts you have I cannot see any viable solution to sort your issue
Just a tip, remember to flush your permalinks after setting