The number of updates (ticks) the game performs per second. UPS optimization is often referring to getting more performance out of the same level of production.
Refers to the act of having one crafting machine's (furnace, assembler, etc.) output inserted directly into another crafting machine with no intermediate belts or trains.
Having two inserters with a chest(or car/ stationary wagon) in between is also considered DI.
Refers to a situation where the ingredient items of a recipe back up because they're being produced faster than they're being consumed.
Can be ideal for performance as it leads to inserters carrying more items per swing.
Refers to the situation where an inserter has swung into its drop target and is waiting for available space to drop the remainder of its items.
Limits the rate of inserter swings by connecting them to the circuit network and pulsing them every so often.
Clocking introduces circuit network overhead, but that can be overcome in some situations by leading to backpressure.
Tutorial by KnightElite https://www.youtube.com/watch?v=kHvqxKLXs54&t=1s