If their description is correct, this has absolutely nothing to do with running scripts in the background.
Also, they are warming the tab when you are hovering over it. If you are just opening up a link in a new tab and not switching to or not coming close to switching to it, it will not 'pre-warm' anything.
Ah. I almost exclusively switch tabs with a trackpad gesture, so this won't help in that case. That being said, switching between tabs hasn't generally been slow for me.
Also, they are warming the tab when you are hovering over it. If you are just opening up a link in a new tab and not switching to or not coming close to switching to it, it will not 'pre-warm' anything.