From Glitch City Laboratories
Lag is the common term for the result of when software attempts to load various processes, but the processing speed of that program is too low. As a result, several instances may take time to fully load, or the system will attempt to terminate other (preferably unimportant instances).
There are a few glitches which take advantage of the game's processing speed in the Pokémon games; perhaps the most well known is Tweaking, which takes advantage of the game's relatively low processing speed to load new map data when the player travels quickly across loading lines.