I think people are missing the real question here. The question is:
How high must a character's movement speed roughly be, to be effectively invisible while moving?
The question is not:
How high must a character's movement speed roughly be, to not reflect light?
A better phrasing of the question, then is:
How high must a character's movement speed roughly be, to be quicker than the [human] eye?
So the answer here isn't about movement speed vs the speed of light at all. It's about what people can see. This is a question that relates to the observer's ability to track the target, not the physical capability for light reflecting from the target to reach the observer. So that if an observer were present while the character moved 200 feet from point A to point B, with a plain open view in daylight, the observer would be unable to distinguish between actual teleportation and very-fast-but-otherwise-normal movement.
For example, a bullet fired from a high powered rifle cannot be tracked by the human eye from almost any angle (you can from parallel to the trajectory over long distances). That's part of the reason that bullets can't be dodged in reality. Therefore, a bullet from a high powered rifle is effectively invisible while moving, and they move at about 3000 fps (feet per second). That's partially because the bullet is small, but either way it's still well below the speed of light.
Let's continue to use human eyesight as our baseline. Firstly because many of our observers in game will be humans, but also because we don't have data about fantasy creatures. Let's also assume, for spherical-cow simplicity's sake, that double your movement is your maximum velocity (i.e., from a double move). Let's also assume infinite acceleration when moving because I'm no longer capable of the calculus required.
There are articles which describe this. In that article, they use an example of kicking a soccer ball faster than the human eye can follow. They state:
In order to become invisible then, the ball would have to go so fast that the brain doesn't have time to process any light reflecting off it. In an experiment, air force pilots were able to recognise an image of a plane that was flashed on screen for as little as 1/220th of a second. With this in mind, we can imagine that the ball would have to be in front of you for at most 1/250th before the brain just never picked it up.
Therefore, any movement faster than 1/250th of a second is indiscernible from teleportation. Since a round is 6 seconds, there are 1500 observations in a round (1500 / 250 = 6). Therefore, the distance you move divided by 1500 is the distance your character can move between each observation from a human observer. And remember that this is all assuming that double your movement is your maximum velocity and assuming infinite acceleration when moving.
For a character that moves 60 feet in 6 seconds (move of 30 plus double move action) this is about half an inch: 60 feet per round / 1500 observations per round = 0.04 feet per observation = 0.48 inches per observation. You'd have to have a move speed of about 3750 in order to move 5 feet and have that movement be indiscernible from teleportation. Double move is 7500 feet in 1 round. 7500 feet per round / 1500 observations per round = 5 feet per observation. In order to move 60 feet, we need a movement speed 12 times that: 45000. A double move is 90000 feet per round / 1500 observations per round = 60 feet per observation.
Fortunately (or unfortunately, depending on your point of view) none of that really matters. RAW is that unless you're hiding, concealed, in cover, or actually invisible, if creature A has line of sight to creature B then creature A knows where creature B is. RAW, creatures have an infinite rate of detection.