Just as other action role-playing MMORPGS, in Path of Exile, farming plays an important part of the gameplay. The loots that drop when monsters are killed are one of the reasons why this genre has persisted throughout the years, and it seems that PoE has done well in keeping with that tradition. Like with most other things, it isn't as simple as picking loot up for farming, because this game has a much different economic system compared to other games. In order to not waste too much time in farming and be as efficient with it as possible, you need to know where to farm, what to farm, etc.
What to Pick Up As you know, quite a few PoE Items are actually valuable enough to pick up, while others should just be left behind. Obviously, the first item type you should pick up are Poe Currency items. For scrolls, you don't need to if you have plenty in your inventory so that you can save a lot of time.
You should only pick up flasks that are good for using, especially those with the "Surgeon's" prefix. Skill gems also should be picked up, especially those with + Quality since they are extremely valuable and if you don't need them, it can be traded for poe currency. You need to get all the rare poe items especially for exalted orb that you can get because if you don't need to equip them, they can be used for vendor recipes. Obviously, uniques are worth picking up too.
Where to Farm Several things need to be considered when it comes to farming locations. For example, you should run through them from start to finish ASAP while having the most number of enemies to flow through. You can do more runs in an hour, and you can get more loot and experience.
Another thing is that you may want enemies that have a little bit of elemental damage, so you have to stack resistances to get hurt too much, and the enemies themselves shouldn't have high elemental resistance, so they can be killed easily. They also should come into mobs so that you can use AoE skills and crowd control on them.