Time is a dimension, but not like space

> Your motion through the x dimension in space, for example, is completely independent of your motion through the other two (y and z) spatial dimensions.

If one considers motion at (or near) the speed of light, that speed would have to be shared among space dimensions, just as with the time dimension. So not that independent.

i've never really 'properly' understood spacetime - can you expand on your comment?

why would going v fast in the x direction affect y and z?

We constantly move at the speed of light through space-time.

If we start to move through space, we slow down through time.

If we go full speed through space, like a photon, we will not experience time at all. So from the perspective of a photon, everything happens at the same time, from the big bang to the heat death.