I have done my best to log exactly what happens when I start 10 both with and without the "Welcome Screen." I need a term for the smaller "New Notebook" "Open..." "Quit" dialog box. I'm going to call it the "Choice Screen" to distinguish it from the "Welcome Screen."
If 10 is supposed to open Welcome Screen, the scrolling text on the spikey stops when it gets to "Opening Welcome Screen." After about 30 seconds a "formatting notebook contents" dialog box appears. After about one more minute a blank notebook frame appears. After about one more minute the blank Welcome Screen is suddenly formatted. At this point, clicking on "new notebook" produces a working new notebook within 5 to 10 seconds, sometimes proceeded by a "formatting notebook contents," sometimes (AFAIKT) not. The complete process takes about 3 to 3.5 minutes. When I close the open notebook, "formatting notebook contents" appears after about 15 seconds, disappears, and then reappears. Then it closes and the Choice Screen appears. If I click "quit," there are no further dialog boxes.
When I try to open the Welcome Screen from an open notebook, I get a "formatting notebook contents" that lasts for about one minute, and then the Welcome Screen appears fully formed.
If 10 is not supposed to start with the Welcome Screen, a notebook appears immediately after "initializing kernels" shows up in the spikey, but the notebook unresponsive for about one minute. During the period, I can't move the window for about 20 seconds. Later I can move the window, but it is unresponsive to input.
When I close the notebook, "formatting notebook contents" appears and disappears twice, but it does seem to be a bit faster than when 10 was supposed to show the Welcome Screen at startup.