I ordered mine in November 2015 and it still works just as fine. Just as fast, if not faster with the Android updates, and the battery still lasts around a day and a half, i.e. usually 20-40% when I plug it in to charge at night.
During the work day, I generally only use it to check headlines at lunch. I also use it to play back mp3s over bluetooth to my motorycle helmet. The screen is typically on during the day for less than an hour.
The 5X is slower than the Nexus 5 it replaced - 50% more cores but they run slower and most apps take longer to start up. It's a mediocre device and the battery life is not good at all.
Right now, the biggest drains, after screen at 12%, are Google Play services, Android System, Android OS and Mobile standby.
I have a theory that it's Google's appetite for data that kills battery life. They make it hard to use the phone with location services turned off or in battery saving mode except when you want a location in Maps. When you want your location, it gets turned on and stays on. There's no easy way to keep it on to a minimum. I think it's a strategic goal of Google to use customer data for everything from location popularity to traffic, and they're willing to sacrifice customer batteries with nudging defaults to get there.
Except that when I first bought the device, in December 2015, it had GOOD battery life. I don't play games, I don't do anything but browse, use a couple of social media apps (as in, two) sometimes, try to keep the display below 40% brightness.
It rotted over time, like fruit left out on a hot day. The battery life became too gruesome to last the day.
It could very well be the massive data needing to be funneled about your commute, which Amazon product you glanced at, which jailed AMP news article you read, which restaurant you just walked out of, and all that other stuff that inspires modern dystopian fiction books and shows, that's draining the battery life.
Our dystopian information-centralized existence killed the battery life on my 5X. I'll take it.
[0] "okay" yes, but if people at Spotify would spend more time on battery and network issues than moving menus to weird places, I would be happier. Alternatives are worst, so it's just a matter of giving them money until they work on something you consider useful. As a consumer, this is infuriating.
My nexus 6 hasn't shown any slow downs etc.. I'm on 6.0.1 and using T mobile. You can pick them up now $260 refurbished or $330 old-new-in-box stock.
What I've done so far is join the Android beta program (yesterday) so I get newer versions of the OS, just in case there's a known issue that's been fixed. I've nothing to loose but my pains.
I found on the Moto X that a Google service was chewing up a lot of power/battery. Upon rebooting, that service would return to being a very minor component of battery use.
The first time my 5x started discharging in similar fashion, I had a look at battery use and found the same thing. Turning the phone fully off and back on, as with the Moto X, seemed to "fix" this.
I'm not saying this is your problem. Unless you never restarted your phone after such an update -- waiting and charging my Moto X overnight was not enough to make it go away. But even if not, maybe this comment will help out someone else.