The new HDR Merge is still taking what I believe is still a very long. The image that took ~12 minutes to complete is now down to ~8 minutes. On my MBPro 16GB this process took less then a minute.
Although there is a RAM ratio of 6 - 16, it still seems that this process could be much shorter on the iPad
iPad Pro (3rd gen) 1T 6GB.
iOS 12.4