Einstein's General Theory of Relativity construes time as a dimension: as one axis of four-dimensional space-time. On this basis, time does not slow down or speed up, because a dimension does not slow down or speed up.
What does slow down or speed up is a process, such as the ticking of a clock. If the ticking process slows down, then there is a longer time interval between each tick; if the ticking process speeds up, there is a shorter time interval between each tick.
This demonstrates, on the one hand, that according to Einstein's theory, time intervals expand or contract, and on the other hand, that time is the dimension of the unfolding of processes. It is because time is the dimension of the unfolding of processes that time has an "arrow" (direction/asymmetry): from earlier phases of unfolding to later phases of unfolding.
On this basis, time is unlike the dimensions of space, because movement along the dimension is a matter of duration only. That is, "travelling" in time is merely the persistence of a process along the time axis. There is no travelling forwards or backwards in time analogous to travelling forwards or backwards in space.
On the same basis, time does not flow or pass, because a dimension does not flow or pass.
What "flows" or "passes" is a reference point, the present, along the time axis. The present is the location on the time axis of the process of construing experience as meaning; that is of conscious processing: sensing or saying. As the process of consciousness unfolds, the reference point 'present' shifts on the time axis, along with what this reference point designates as 'past' or 'future'.