Planet earth calling @dzo, over...
I've spent the last month completely rewriting the HDR code and I think it's much better now.
There's not a single line of code left from the old HDR, and it now uses Halide (http://halide-lang.org/).
I've also updated the Low Light processing (Camera2 only) to align and merge up to 32 frames. You can use it in good lighting conditions for noise removal too. 32 frames will be slow to process so 8 or 16 is more sensible most of the time.
The new code runs great on a Nexus 5 but is noticeably slower on a 5x, I'll look into this but it's probably a hardware limitation.
There may well be bugs in this new release, it's still a bit experimental.