It's worth noting that with HDR+ enabled on the Nexus 5, you can no longer enable the flash or timer. And the processing time can feel frustratingly long. So leaving it enabled all the time doesn't work for everyone.
Not sure if there will be a change in the processing itself (bar filling up afterwards), but with Lollipop taking the picture (circle loading) should be a lot faster. They also say it's 0.3-1 sec, which isn't too bad. Normal photos used to take that long on high-end phones a couple of years ago.