The speed shirt was introduced in Smarty Pants.
The episode begins with Theresa giving Alex, Justin and Max a wizard lesson as Jerry went to the 'No Wizard Left Out' convention. However, she soon gives in when Alex and Max argue over what side of the chair is theirs. Justin then takes over by saying "well she lasted a lot longer than Dad said she would".
Justin then opened the cabinet containing the Speed Shirt...anybody wearing it instantly gets supersonic speed...Justin demonstrates this by making a sandwich from the SubShop.
It is introduced along with the Smarty Pants.
The speed shirt is technically a jacket, which is orange with yellow on it. The jacket activates once fastened.