It's always slow, especially the MSDN web sites. Why?
Below is a screenshot from the document "Office 365 for IT Pros.201611"(Page 73). It explains the passive authentication flow, which is the default user authentication mechanism for web browsers.
In theory, after log in, the user credential/ticket is cached, and the rest operation should be faster. But unfortunately, there are a lot of dynamic data loading on MSDN, which may triggered other authentication events.
Maybe that's the reason.
PS: why Google web sites are much faster?