* 더 빨리 (deo ppalli): This is the most general and widely used way to say "faster." It literally translates to "more quickly." This is suitable for most situations.
* 빨리빨리 (ppalli ppalli): This is a more emphatic and urgent way of saying "faster, faster!" It's often used in a commanding tone and can sound a bit rude if not used appropriately. It's best used amongst close friends or in informal situations where urgency is paramount.
The best choice depends on the situation and your relationship with the person you're talking to. For most situations, 더 빨리 (deo ppalli) is the safest and most polite option.