when i on the HDR in LUMIA 640 XL in OS WINDOWS 10 it takes approximately 2 minutes in saving the captured pic and during that picture saving i cant make another picture.
2 minutes are too long. Mine takes less then 30 second most of the time in HDR.
The problem may also lie in your sd card if are using a slow/fake one(rip offs are common these days) and have set the option to save pics in sd card.
Or maybe you have less internal storage available or there are too many apps open in the background.
A fresh install of the OS could fix it along with other problems.
I also had this problem. But it's a lot less often after a fresh install.