Think of two cars running side by side on the highway at 60 mph There is NO relative motion between them. Should one veer slightly away, then both drivers would see relative motion.----The more veer angle, the faster the relative motion. ALSO the driver who stays on the highway will see the other driver drop BACK behind him on the highway. (assuming the cars do not speed up)
You can think of TIME as the inherent motion in all matter at a function of Light speed in the direction of the 4th dimension. Two particles side by side would be relatively "at rest" in the 3D context---even though they are moving Parallel in the 4th D at a constant c / sqrt 2 speed. Should this vector angle between the two (as a consequence of energy) increase slightly, we would see RELATIVE movement between the two objects in the third dimension.----Just as the cars on the highway, and ALSO one particle would drop Behind the other in "time" (Rather than the highway) from the POV of the other particle. Since the motion speed through time is constant.
Therefore we can see that the time "slowdown" or "Drop back" is a simple geometric result of the veer angle between the two particles, just as car on the highway tends to drop back with relative motion. ALL motion in our 3D world is Relative motion arising from a constant motion in the 4D direction inherent in all matter we call TIME. We don't notice this 4D motion as we cannot sense the 4th Dimension.
This "Time motion" is c / sqrt 2 , so relative motion at 90 degrees would = "c" , Light speed. Beyond 90 degrees of veer, the other particle would be moving relatively "Backwards" in time to the other-----so there is NO increase in relative speed---Meaning c is a natural relative "speed limit" .
Of course, with the car example, the maximum angle would be 180 degrees and maximum relative speed would be 120 MPH---above 180 degrees, the relative speed will tend to slow down again
---But the highway and time are really two different things and involve more dimensions, but the general principle is the same.---The relative speed of Time driven particles and cars running the SAME speed will have to max out at some angle, depending on the dimensional context. In a 3D system it is 180 degrees and a 4D system it is 90 degrees (relative to our POV). Simple geometry and makes perfect sense if we think about it.----Relativity is really simple after all.