Loot Priority Order
The most common mistake new players make is spending too long looting. Efficiency is everything. Your loot priority should be: weapon, armor, healing, then attachments. A fully kitted weapon with no vest is worse than a naked gun with level 2 armor.
Tip
Use the Tab inventory screen to drag-and-drop items faster than pressing F on each one individually. This alone can cut your looting time in half.
Items You Can Skip
Not every item is worth picking up. Skip pistols (once you have two primary weapons), level 1 gear when you already have level 2+, and excessive ammo (200 rounds of 5.56 is plenty). Bandages are inferior to first aid kits — drop them once you have 3+ first aids.