The overall drop rates are pretty poor so farming the right activity is the most efficient. If you have at least two in your fireteam then the Shattered Throne final boss is the ticket, save the CP on a secondary character and use it to farm with a friend or two. GL the knights and a Titan consecration or two takes them out then just 2 shots from Wardcliff Coil the boss, rinse and repeat for 21 to 22 essence a pop. Takes around 2 min per run including loading in. Expert lost sectors - good for solo depending on sector but basically everything is fine apart from the Pale Heart sectors run as a team to speed things up and if you aren't bothered about exotics. (Note Master does not give and more essence so don't waste the effort) 20 essence per run and takes around 3 to 6 min running solo depending on sector. You can also select the Devils Lair strike from the node in the Cosmodrome which is easy to run in 5 min. Rewards 8 essence per run.