It is not well known that the Theory of Relativity is almost misnamed. Relativity was well known in physics since Galileo Galilei. That is, the relativity of space was well known. With Albert Einstein’s derivation of the Lorentz transform, the relativity of time was introduced. But the relativity of time was not of the same order as the relativity of space since it required speeds approaching that of the speed of light.
Nicholas Copernicus wrote in his 1543 book, The Revolutions of the Heavenly Spheres: “Every observed change of place is caused by a motion of either the observed object or the observer or, of course, by an unequal displacement of each. For when things move with equal speed in the same direction, the motion is not perceived, as between the observed object and the observer.”
Galileo in his 1632 book Dialogue Concerning the Two Chief World Systems noted that a person below the deck of a uniformly moving ship has no sense of movement and so objects dropped fall directly toward the feet. He articulated a principle of relativity: “any two observers moving at constant speed and direction with respect to one another will obtain the same results for all mechanical experiments.”
That is, a Galilean inertial frame of reference is in relative uniform motion to absolute space. This leads to the notion of absolute time and relative space: although different observers on different reference frames see space differently, they observe time the same way.
Einstein derived the Lorentz transformation from this foundation, allowing measures of time and space to affect one another. This solved the problem of electromagnetism and the result of the Michelson–Morley experiment.
One could say that time in the theory of relativity is relative in a minor way, whereas space is relative in a major way. The relativity of time requires speeds approaching that of the speed of light in a vacuum. The relativity of space is true at any speed.
But note one could modify Galileo’s statement to say that any two observers moving at constant pace and direction with respect to one another will obtain the same results for all mechanical experiments. In that case, time is relative at any speed, too. The key is to measure motion with pace, rather than speed, so that the independent variable is space. That allows time to be relative and have multiple dimensions.