I think of it like this-
We’re all moving through 4D spacetime at the speed of light. When you’re sat perfectly still you’re moving through time at the speed of light because you’re not ‘using up’ any of your allocated speed to move through the three physical dimensions.
If/when you go charging down a hill, or grinding up it, or do some sweet jumps, you have to use up some of your ‘speed allowance’ so you in fact end up moving through time a little more slowly (relative to an observer).
If we’re moving through 4D spacetime at the speed of light (300,000 m/s I think?), if we use some of that speed to move through the three physical dimensions at 100 km/h or 28 m/s (in a car for example) then we’re still moving through spacetime at 300,000 m/s, but moving through time at 300,000 – 28 m/s = 299,972 m/s because we’re ‘using’ 28 m/s to move through 3D space.
I’m probably way off the mark and missing a square of something somewhere, but that’s how I think about it.