Skimmed this thread. Glide is another name for loading screen. Apologies if it has been mentioned,
As for too fast for orbital. Man that's just daft, Insta drop out and survive. You should be faceplanting the planet.
And as per the OP Take it in well below the blue zone. you kind of get a feel what is too fast after a while.
I think it is certainly a frame of reference mechanic. Others have said you can see points of interest etc on way down, but I think the final rendering is trying to work out exactly where you'll "pop out". In normal space. This then let's the graphics engine draw the scenery correctly.
So it's part loading screen, part guess mechanics. ... I guess