T-grain films, such as Delta and Tmax, when used in conjunction with developers like Tmax developer, have much shorter developing times then HP5 or TriX. A developing time of 7 - 8 minutes makes it much easier to finish a roll of film in a single class period. With many developers HP5 and the like can take 13 -15 minutes. This is one of the major reasons high school photo teachers favor Delta and Tmax.