The pull request says systemd 257 and later asks the console for its size when the first process starts, and waits for the answer. With Plymouth holding the screen, that answer sometimes never comes. The argument systemd.tty.term.console=dumb tells systemd to skip the query. emirb wrote that nothing visible changes under the quiet splash screen.

emirb wrote that the counts came from repeated boots of one install in QEMU. emirb wrote that, without the argument, a prebuilt image hung three times in 30 boots and once in 40, and that a stock early-boot image hung seven times in 40. With the argument, emirb recorded no hangs in 80 boots. emirb wrote that those 80 boots used the installer, and were not repeated through the migration that rebuilds an existing machine's boot image.

The merged defaults file carries the argument for a fresh install. A one-time migration rebuilds the boot image on a machine whose current command line lacks the argument and whose defaults file has it. It marks itself done only when a boot entry actually carries the argument.