I am going to guess its getting the network up and going. You should be able to hit escape on the screen and see where the boot is in its process. Getting the networking going during boot is something a lot of linux installs will do as most enterprise/devs (tbh the biggest part of their audience) have network attached storage. I have never looked into getting it just to move on past it (it will still start the process for getting networking online, it just shouldnt pause) I know that some fedora installs I have had in the past did this.
But boot it again and hit escape and get more info and if it something else post again here maybe we can help better.
While this sucks, I am thinking they are seeing how well the proton project has come along and are thinking not spending the resources on something that is already a great wrapper (and arguably part of the reason they pay that % to steam for, so it would be seen as a already paid for thing). I would like to hope we will see greater polish and depth with the savings.