This paper has lots of info about HBM. They explain CAS latency in page 4:

"Demystifying the Characteristics of High Bandwidth Memory for Real-Time Systems" https://upcommons.upc.edu/bitstream/handle/2117/362010/HBM.p...

> For this CAS latency, HBM (Observation 6) offers a significant advantage over regular DRAMs. This is because HBM has tCCD = 1 or 2 compared to tCCD >= 4 for DRAM. Accordingly, HBM can reduce the CAS latency component to at least half of its DRAM value.