Hi,
Start and End routines are just placeholders for you to enter custom code; neither of them is inherently slow or fast. It all depends on how you use them.
For instance, if your business rules say that certain data needs to be deleted/discarded while loading, I'd try to achieve that in the Start Routine. That removes a good chunk of data right at the beginning thereby reducing the amount of data going into the field rules and the end routine. Beyond that they're fairly interchangeable from a performance standpoint.
Regards,
Suhas