User account invisible on start up I've just purchased a built to order MacBook Pro 15" Late 2016. It has been showing faulty behaviour since the first bootup. 

Firstly, right after the Apple logo on startup, the Mac gets extremely slow, like you'd need a minute or two just to move the cursor to click the user you want to login as.

Secondly, on my first bootup I created a second user account. So (User1, User2 and Guest). However when booting the Mac it only shows User1 and Guest. User1 his avatar is like a black version of the guest icon, very strange. Once I log into User1, it will show the option to switch to User2.

I've also had a couple kernel panics. (Sorry didn't save the output). 

I've ran Apple Diagnostics several times and it shows no problems detected.

Any idea what could be the cause?
It's hard to know what slow means. If you have FileVault enabled - only accounts that are cleared to unlock e drive show up after the boot to the pre-start screen. 

Once one of those users authenticates, the volume gets unlocked and then the OS boots. If you don't expect this layer and seeing the OS boot after user credentials are presented, that might explain everything. Without a stopwatch to time what is slow, it's hard to tell more what you are seeing. 

Since all new hardware gets AppleCare for questions like this, I would get their help narrowing down your variables and perhaps ask a follow on question with some specifics if needed.