We should not simply add speeds or velocities since that would not conform to the measurements made of phenomena in the universe we inhabit. The laws remain invariant under transformation. Anyway, time is an illusion and should be removed from the equations by change of variable.
Time is dimension. The meaning of that dimension is "is", or "exists". What exists is energy and exists in this world with an uncertainty within E~=t/Hbar. It's never an illusion, or appears to be so in the local frame. Einstien cleared up any misunderstanding that existed before that.