It is not recommended to exceed 2,000 items in a list level, and to use Explorer view to view large document libraries. Instead, you may choose to use All Document view, when open a SharePoint document library in Explorer view, placing the pointer over any of the enumerated files requests metadata for all of the files in the folder that you are browsing.
As in this scenario, it may be the large number of the documents that make the Explorer View not working, here is an article about “Tune Web Server performance(Office SharePoint Server)”, there are some points we need to pay attention to in “Train end-users to work with large files”, for more information, please refer the below article:
Thanks & Regards,