By saving your kills for elites, you're maximizing the amount of rolls you get per Rift, saving you Rift Fragments per legendary acquired. Additionally, while in Nephalem Rifts, your countdown timer is based on the number of monsters slain. While in Diablo 3 originally there was a slight benefit for killing every monsters in the form of extra rolls on legendaries, with Loot 2.0, there is a built in system that ensures players get approximately 1 Legendary per hour, meaning that time spent on these weaklings is time wasted. Though in some cases it's unavoidable to face some trash mobs, you can quickly clear them out before getting back to your farming.

Non-elite monsters make up the majority of what you'll be seeing in both Bounties and Rifts, but if you're on the correct difficulty (see above), it should be relatively easy to blow through these unimportant mobs to tackle elites and champions. A good metric for knowing you're on a good difficulty is being able to one shot trash mobs, allowing you to quickly plow through the unimportant monsters to get to what you actually care about- the elites. Farm on a difficulty that allows you to quickly move through packs without needing to slow down or focus on their individual abilities.

Taking on a difficulty that's too much for your character is a good way to cut down your farming speed substantially, and you'll increase your repair bill besides. When you're trying to farm effectively, don't try to be Superman.

Diablo 3 reforge legendary low level