It depends on the hour. Recently had to do a T3 to T1 transfer at 8 a.m. Actually took the bus from T3 to T1 but after sitting on the bus at T1 waiting to disembark for something like 15-20 minutes and faced with a line that went down the escalators we ended up ducking the T1 transfer security headed straight out through passport control and customs and then came through the infinitely faster main check in security. Next time I plan to skip the bus altogether.