Attempting to open the Minecraft launcher over slow internet can result in complete inability to play the game when offline play was previously available.
The Microsoft sign-in time out settings seem to be extremely temperamental resulting in failure despite there being some small amount of internet connectivity. This alone is unacceptable, as other services on the device were actively performing downloads at the time, whatever data that was required for sign in should have been able to get through eventually.
This however results in a much more critical issue. Failing to sign in under partial connectivity puts the launcher in a state where it requires internet to ever work again. Stating you must sign in to restore offline play. It should not be possible to brick a previous ability to play simply by making the mistake of being connected to poor internet.
Environment
Windows 10
Same here, can't connect to the game just because “slow internet”. I was trying to play with friends, now I have to wait til it's fixed