As per the 5G NR numerology the number of slots within a frame increases as numerology increases.
Duration of frame is fixed in 5G as well as LTE which is 10ms.
if we see at the table above, the number of slots for mu=0 are 10 per frame and for mu=1, 20 slots per frame.
So compared to mu=0, mu=1 slot duration of each slot has to be halved to accommodate in 10ms frame duration.