Spigot is the most widely used modified server software for Minecraft servers in the world. It is the engine of many of the most prominent Minecraft server networks to ensure that they can cope with their high number of users and ensure the satisfaction of their players. Spigot works by reducing and eliminating many causes of lag, as well as adding useful features and options that make your job of managing your server much easier. The end result is that your server pulls ahead of the competition and you no longer have to spend time pulling your hair out of your head over players complaining about lag.
It should also be added that Spigot is perfect for smaller servers, especially due to features such as cancelling CPU usage when not in use, saving power, and other resources when there are only a few or no players connected to your server.