The LAG is an NPC who wanders around Ankh-Morpork. Unlike most NPCs, the Lag will respawn immediately or almost immediately after being killed, appearing in some (apparently random) part of the city. Its equivalent in Bes Pelargic is The Laggy-san.
If you shout out, "Lag", while in Ankh-Morpork, the LAG will shout something back, and soul at you.
Yes, this is the real thing, the lag that always bothers YOU! Kill it so you can shout "I killed the LAG!". It is in good shape. It is standing. On its chest is a handwritten sign which reads: "Will LAG for food". Wearing : a "Will LAG for Food" sign and a gold ring.
The sign it wears:
This is a sign that used to belong to the LAG. Something about it seems heavy and ponderous, though it doesn't appear to weigh that much. It appears to have something written on it. It is in excellent condition. > read it You read the "Will LAG for Food" sign: Written in flowing script: Will LAG for Food
This ring looks like any other ring, but feels strange to the touch. It appears to have something written on it. It is in excellent condition. > read it You read the gold ring: Written in flowing runes: Three rings for the Deadly Pings behind the screen, Seven for the Packets Lost and Never Found, Nine for the High Speeds, rarely seen, One for the Great Lag on Discworld ground In the Web of Servers where the Routers lie. One Lag to slow them all, One Lag to scare them, One Lag to stall them all and in the net ensnare them In the Web of Servers where the Routers lie.
Wearing this ring will give you the message, "You feel like you have trouble moving..." and removing it will give the message, "You feel able to move at normal pace again.". However, it's not clear whether it actually slows you down at all.