its as fast as GDDR 5, and probably GDDR 6, but the difference is the amount of power required to run hbm2 vs GDDR 5. hmb2 is a low power memory.
for all in one vr headsets like the quest from oculus, a low power gpu memory means more battery life and better gpu's used.
whats keeping hbm2 from being used is the market from nvidia and amd isnt using hbm2 because the startup cost to mass market the new memory is expensive, but once hbm2 is being mass marketed and replaces gddr 5, and gddr 6 the cost of hbm2 will go down to gddr 5 and gddr 6 price levels.
so since nvidia is pushing for driving and ai and oculus quest as is amd, its in their best interest to switch over to hbm 2 memory for the reasons i listed. think of it as a shift similar to ssd using nvme over m.2 instead of sata.