Got my Nexus 5X, flashed latest Resurrection Remix (includes a custom kernel and root) ROM to it. Installed all my apps (~50), none of which is a messaging or social app except Twitter, which is the only one I use. Email is set to sync every 15 minutes.
No lag, no issues. Even with a few apps open in the background and with browsing (Firefox, with uBlock Origin installed, though AdAway is also enabled) I have ~40%/600MB RAM free most of the time. I could only get the SoC to throttle with screen brightness cranked up and after running several 3DMark tests consecutively (averaged 1060 in "Sling Shot using ES 3.1" without throttling, ~900 after); it hasn't heated up or throttled otherwise. At idle/low usage the cores are at 30-32C.
It wouldn't surprise me at all if the people experiencing lag are using Facebook Messenger or maybe some combination of resource hungry apps, or maybe perhaps vanilla Marshmallow has some specific issue on the 5X. I'll report back if I experience any lag or slowdown, but so far so good. Fastest Android device I've owned so far and love it overall. Great value for $300, even if it's not quite as premium (it's very light - lighter than my 2013 Moto X, which is smaller - and plastic, but it's a decent matte plastic) as the $600 and $700 phones. Only minor annoyance is the vibration motor, which feels/sounds a little cheap and chirpy, so I disabled it for screen presses. Too early to tell about battery life, though I expect it should last long between charges (very little drain so far on standby, though). I have to test the camera more, too.
I ordered a Project Fi SIM, which should be coming in tomorrow or Monday. Ordered a case, as well: