This is super ironic, I just got on here to post about the lag/slowness I've been seeing on my 5x in the first few days.
It's totally fine when you first unlock it and runs great, but after a few minutes of browsing around and using some apps like Facebook it gets quite warm to the touch and starts running super slow. Then there's the camera app that runs down to 5-10 fps in the viewfinder at times and stutters while taking photos even with HDR turned off. It's really got me worried about it.
At other times you can open darn near every app on the phone, hit the multitasking menu and it's buttery smooth 60fps and resumes apps promptly. But if you keep using it for a while it gets slow again. For instance, using the stock camera app and recording a video or even taking a single photo then going back to the home screen can cause the whole launcher to reload along with Google now. That's a bit embarrassing. My 2014 Moto X was perfectly fine doing that.
Basically as long as it's cool and not being throttled it's happy, but after a bit of use it down clocks and is a mess. The SD 801 phones I had didn't seem to have this issue, like the Moto X and HTC One M8. Both were super fast and the only reason I left was because of the cameras on both. M8 lacked resolution and blew everything out and the Moto was pretty bad in low light. That's one upside of the 5X, the camera (not the app but the photos themselves) has been superb. Pretty much on par with my 6 Plus when taking photos side by side.
I'll give it some more time and a few reboots but if it's still bothering me I'll send it back, as much as I love Marshmallow and the size/build of the 5X, the UI lag on my 6 Plus is starting to not look quite so bad. Dropped frames aren't as big of a deal as having your home screen reload and app switcher take 5 seconds to come up.
Oh, and I can easily go from 100% charged to 30% by 3PM, where my 6 Plus wouldn't get that low till about 9PM. Smaller battery I know, but worth noting. Luckily it'll regain half it's charge in 20 minutes or so. Pretty much the same usage too.
* One more note. Regarding charging, my model produced this month has no issues with the touch screen while charging like some of the launch units. Performance seems to be the same too.