Cancelling current production should be done just as urgently. If I'm at 23/1000 with my cavalry training, hitting cancel should either (a) instantly finish, giving me 23 cavalries and refund of all the materials for the other 977 or (b) finish currently trained unit and in few minutes give me 24 cavalries + refund mats for 976. Same applies to PH. However, if cancel feature is omitted for now, easy workaround would be that if you queue 1000 recruits, game actually just treats it identical to queuing 40 x 25 recruits. Note that this is only ok with barracks. In PH, we really want bigger stacks since they go to star menu and need to be used manually (or sold).