Not that it's much consolation, but you can make your Nexus 5X run better by rebooting it nightly. I am hoping future software updates fix it, seems to be some sort of RAM issue.
I've had the camera spend 5-10 seconds to launch, then spending 3-4 seconds per picture ... which it ended up not saving.
Android 6.x has some serious memory-management issues as far as I can tell, because the Nexus 6P owners I've talked to online does not have these issues, but they have 50% more juggle room for memory leaks and to work around memory-fragmentation issues.