The game has always been slow. I remember teleporting or rubberbanding/self-healing enemies back in D2's first raid, The Leviathan.
There has always been issues loading menus, albeit better at times, it's still been a launch issue.
Not saying that it's not getting worse, or doesn't need to be looked into, not trying to be negative, but I don't think it's going to get much better.
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Grain of possible positivity though (and salt); They announced "No More Vaulting" back in Summer with Lightfall's announcement. At today's server issues, it means either of 2 things: They're high on supply and the server rooms will ignite shortly after Lightfall launches, or they've finally invested money into their server hardware / architecture to be able to support that claim for Lightfall + Final Shape.