This tutorial shows how skybow rich forms can deal with large lookups with more than 5000 Items. Deal with cascading lookups.
Read this article to understand how to configure SharePoint to manage large lists and libraries in general:
https://support.office.com/en-us/article/Manage-large-lists-and-libraries-in-SharePoint-b8588dae-9387-48c2-9248-c24122f07c59