So we are seeing an issue that I wanted to see if other folks are also experiencing.
When a new Touchbar MBP is FileVaulted and has an Apple USB-Ethernet adaptor plugged in either via a USB-C to USB-A adaptor or one of the Apple Multiport HDMI/VGA adaptors, the filevault login screen will be extremely slow/unresponsive and often won't let you even type anything into the password field. If you restart without it plugged in, everything is fine.
We've noticed that there might be an issue with the Apple USB Type C Multiport adapters. Users with one of these adapters and an external display have experienced interruptions to the external display (i.e., black outs) that last for a second or two. These can occur in several series of black outs or just one. Happens for a few seconds then doesn't occur again for a while. Notable that the main laptop display does not sleep/black out.
We don't have many of the other adapters to test substitutes and obviously no Type C monitors are readily available. Testing with other types of dongles/adapters will help tremendously.