I decided to finally try SM 16 after about a year of hearing others suggest avoid SM 16. I had hoped most of the major issues would be resolved by now, so far it seems fine. I offer for ST to measure something though:

 

Log in to a SM 15 site with "remember me" checked so you don't have to login the next visit. After a period of time return to the site and time how long it takes to get to your inbox (or start page) i.e. access the content.

 

Now do the same in SM 16. My guess is it's 5X longer to get to your content. 

 

In SM 16 there is some "Welcome Back [NAME]" dialog and some processing going on, not sure what or why. I suggest measure this and improve it with the goal of getting the user to their content as fast as possible. Bypass the welcome dialog, if someone thinks this is a looks cool idea and welcoming someone back, we don't want it. We want our content as fast as possible.

 

I look forward to ST's response.