The easiest way to picture the situation is to imagine that the Earth is a stationary object. The ISS flies at a certain height and the distance it travels in one orbit is the circumference of that circle.
So you know the distance (ie far one orbit is), and you can easily time how long one orbit takes.
And the speed in kph or mph is simply the distance travelled in one orbit, divided by the time it takes.
(In fact the height of the ISS varies between 370-460 km) above the surface of the Earth, and the speed varies between about 7.7-7.6 km per second).
You could make a case for the ISS travelling at vastly different speeds, and it is (for example if you factor in the rotation of the Milky Way, or indeed the speed of the Milky Way) but those factors are not relevant as whatever other components there may be in the motion of the ISS, still in the orbit it is in, it must travel at the speed it does in order to stay in orbit.
Your perspective on Earth isn't important either. If you are on the North Pole then your speed with the rotation of the Earth is nil (you just rotate once every 24 hours as the Earth spins); and if you're on the equator then you are rotating, with the spin of the Earth, at a bit over 1,000 mph. But that's not relevant to you, because in reality we are all stuck at ta point on the surface of the Earth, and what speed our rotational component is, makes no difference to us.
) The ISS is a white light but is moving across from West to East at quite a speed.
