When you say “the next time”, do you mean the next epoch? That would explain exactly what you describe (faster computational time, lower loss)…
After the first epoch, if your dataset fits in memory, the loading times will be much faster if you are on Linux, because of the system caching system.
However, if you are talking about launching a new experiment with the same script, there is no justification for the much lower loss or faster loading times that I can think of…