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.
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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.