GDDR5X will not be as fast as HBM2 but promises to offer a much better price/performance ratio, making it better suited for applications where price is a concern. Transitioning to GDDR5X promises to be a simple process and this new standard promises a significant bandwidth increase versus today's GDDR5 memory.
Why does any of this matter? GDDR5X memory pushes data rates to 10-14Gb/s. That's up to double the memory bandwidth of today's GDDR5 devices, and the best part for manufacturers is that it's not a completely new design—the memory architecture is largely the same, which will make transitioning to GDDR5X an easy and cost-effective affair.