Huh?
On a flat Earth, the Earth is not in motion. I fail to see why any speed is "needed." Neither flat Earthers know (or care) nor do round Earthers know the actual dimensions of Earth. Nor the actual dimensions of the sun. These are educated guesses using math.
Educated guesses, while good, are not infallible. You cannot take a ruler to the sun and not have it burn. Heck, you can't even walk closer to the sun. Although in westerns they show the man walking off into the sunset, if the camera tracked the man rather than the woman, they would never get closer to it.
Second, as I've tried to tell you, perceived reality is like a diorama show. You swivel in light to bright the scene and the figurines in your diorama don't ask how big the flashlight or how fast it is going. They are stupid questions. The only good question is if the sun's angle looks right. If it doesn't, you have to adjust the position.
You have this idea that the sun has to be universal, that it has to make its way across the world and back in a single day (like Santa!) but while there is a local sun, we humans don't actually know the measurements, so we just throw numbers at it.
Science is about knowing things, using observation and methodology. But if you can't exactly hold something in your hand, at the end of the day, all you have is assumptions. So telling us that we "need" to think that the sun goes this speed or that is based on us accepting your physical dimensions for Earth.
I, the doll in the diorama, envision reality as a large circular hangar with Some sort of light show that I dimly understand. I know that the light angles in to my diorama, or others, and while there despite the zone that I can see being pretty tiny (if stars seem fainter now than my youth, I rightly blame nearsightedness, not the stars dying off), it appears to linger there for twelve hours. But given all of the other zones, I suspect that this is not actually the case. We are seeing an illusion, and the Magician is not telling his secrets. We can come up with an explanation (zone of perspective is photosensitive using relative angle),but since we can't stare at the trick without going blind, like that truck thing, it's probably better for my health to just say "I dunno."
Depending on my height, the perspective zone can be anywhere from about five miles to thousands of miles (going somewhere above where we can get without compressed air). Yet if I were to sit in a infinity-powered helicopter, and watch the sun rise and set from 25,000 feet, I should not expect the day to be any longer. But AI when asked this question, assumes the curvature, and gives a crap answer. Turns out when I asked about Everest instead (about 4,000 ft above that), they gave me more specifics. The day is longer, but not significantly so. 15 minutes more sunlight than at sea level. You're comparing a view of hundreds of miles on a clear day to maybe five or ten with alot of good atmospherics, yet only getting fifteen minutes more actual sunlight? Clearly our perspective cannot be measured in terms of a math equation, or we'd have days be ten times as long at high altitudes. That doesn't happen.
We are watching a magic trick. The laws of perspective tell us more or how the trick works, but we don't pretend to know distances or speeds. You'd need the schematic for the trick for that.