Time is this simulation/universe's equivalent of FPS, and the reason it apparently slows down for particles as velocity increases, or when in close proximity to a very large concentration of other particles that mutually exert gravitational effects on each other, is that it's very computationally heavy (fewer forces act between particles at a distance) and there are limits imposed on the simulation to conserve the resources of the CPU running everything. More specifically, one of the hard limits is that nothing travels faster than C, and the closer you get to C the slower your particles are rendered to compensate for all the extra interactions with other matter that need to be calculated when you speed past them.
Furthermore, when too much interacting mass is concentrated in a location, the mass is condensed into a singularity and reduced to an almost featureless point that is much easier to render, with only it's mass and gravitational values preserved. As other particles enters the vicinity, although not close enough to be added to the singularity and essentially removed from the simulation behind the event horizon, the clock speed at which they are rendered is reduced.
