Hey there. I've got my Nexus 5X two weeks ago, and I have been experiencing issues with it, however, I believe they are entirely software related.
Just a little while ago, my phone got in a terrible state. Something was using the Location constantly, and the CPU was at 100% at all times. The phone slowed down to a crawl, and I had to eventually reboot it.
All that has led me here to the forum, to see if fellow XDA members have found the culprit. And, judging from the other posts, I seem to be right, and this seems to be entirely software related, and related to some app that I have installed - though I'm not letting Android off the hook yet. I'm in a bit of a problematic situation, as I have no means of connecting my phone to my computer (I haven't bought an USB type C to type A cable yet), so, I can't root and investigate further right now.
All in all - I don't really worry. I'm sure all of this can get fixed. I'll try a new kernel and a process monitor as soon as I find a way to root my phone, get a custom recovery on it, etc. But yes, as of right now, there seems to be something misconfigured with the Nexus 5X, as I hear no complaints from my friend who has a Nexus 5 running Marshmallow, and as far as I have read - no complaints from Nexus 6P users, either.
Will try to update you if I find any data!