Why does video game footage on YouTube not lag but the actual game can lag

because the game footage is played on a newer platform i.e. a newer ps3 or xbox360 or pc. or it could be that it was played with a faster broadband speed.

When a game is being played, the computer is working hard, performing millions of calculations to control what is going on. Sometimes there's just too many things happening, the computer slows down and you get lag.

However a recording of the screen is different. Rather than have all those calculations going on behind the scenes, the recording is just a series of static snapshots which give the illusion of motion. This won't lag under normal circumstances, again a slow computer might struggle to play a huge video file in high definition.