This article describes an issue where there are a lot of requests to https://dl.episerver.net/version.json which take really long to response. 

This following blog post should resolve the issue. 

http://dmytroduk.com/techblog/customizing-how-statistics-tracking-scripts-are-loaded-on-pages-when-using-episerver-find

The UI is downloaded from dl.episerver.net on the server and cached. This means the application can deliver the resources from cache for performance by delivering them directly from the server. 

Also, remember to use the appropriate version as the version number when downloading the files:

https://dl.episerver.net/12.4.3/epi-util/native.history.js
https://dl.episerver.net/12.4.3/epi-util/find.js