when you’re talking about efficiency, you’re primary factor is going to be time. You want the highest % chance in the lowest amount of time. Which actually means not killing anything and just looting troves is one of the most “efficient” ways to farm legendries… 
The only way killing trash mobs is going to help your efficiency is IF you can kill them fast and you don’t go out of your way to kill them. if you can kill them as your run to the hero/boss/trove and it adds little to no time, then kill them as it will help! If you have poor AoE and have to stop and fight them where you’re adding even 20 seconds onto a 120 second run, you will lose efficiency. The blanket statement of killing trash mobs is a great source of legendries is misleading as it is case specific… They can drop them and you should kill them, especially during leveling. But if you’re doing speed farming runs and you can’t kill them fast, then skip them.