Time is one of the most objective variables we have. It depends only/fundamentally on the speed and gravitational field the measured body has/is under.
There is a number of regular processes that require the same amounts of time to complete.
Your slow or quick motion wouldn't be perceivable for the observer subject to the same alteration, but it would be visible to someone outside of this frame of reference. I'd say your slow motion is what Einstein called relativity.
It's important to note that relativity isn't arbitrary, it depends on measurable distances, masses and speeds, as well as detailed densities, rotational periods of measured objects.
Maybe, we shouldn't look at time separately, but think of the totality of speeds, masses and distances. Or maybe we could calculate a time of an object with minimal mass and minimal speed (could be 0, but might be impossible) and make it our universal clock. Problem with that is, there were times during expansion of the universe when there was no empty space, so maybe a better solution would be to calculate a composite center of mass of the entire universe and how much time passes for it. This center of mass is constantly changing but this could be incorporated to the time function as well using a variable of speed. This is interesting to find, but not helpful for measuring time, because time is measured with a respective goal, similar to our earth-solar time, I wonder if time in this sense without a goal would be more objective. I'd be more inclined to consider many simultaneous times, depending on the situation.
Time difference in a general sense may mean a diversity of:
-the time of beginning of a process/object compared to another, its duration
-the speed of the process or object
-the mass or distance of the object