My thoughts are along the lines of these statements. But I would expand a little more.
When I shadow box, there's no difference between me shadow boxing and me actually trying to do the same combos in a real fight. My shadow boxing movement and strikes = my fighting moves in strikes.
I see open hands, and other movement that make me question if you could or would actually do those same movements in a fight. I don't know how other people train shadow boxing, but for me, anything you see me do in shadow boxing is something that you'll see me pull off in in sparing or competitive fighting.
My personal opinion about shadow boxing is that people often try to do too much in the beginning, Start with one punch shadow box your jabs, or shadow box a 3 hit combo. Start small and build upon, that. Speed is important but not that important in comparison to getting other things in order. Once you have to "other things" then you can increase the speed when shadow boxing. For example, once I get my combo working and everything is connecting, then I can start to speed things up in shadow boxing.
This is an example of me shadow boxing. I will work this technique out every day until I can go left and go right. The shaking of my arms is not habit. I'm naturally stiff in the shoulders so I'm always trying to remind myself to relax more. I often use shadow boxing to figure out things out and to figure out which combos can lead into other strikes
The speed that you do shadow boxing will vary depending on the focus of the training.