I've had a Nexus 5X since they came out, and recently got another one for my daughter, taking advantage of the discount that comes when you sign up for Google Fi.

Screen: it's a traditional backlit screen. You don't get the deep black levels of an OLED but you do get very good color accuracy. The thing I hated about my old Galaxy Nexus was that they had the color saturation cranked up to ELEVEN, so everything looked garish. (The latest 1+1 news suggests they're releasing an SRGB mode. Android doesn't yet have support for multiple color spaces, so apps that want to take advantage of the wider color gamut of OLED displays would have a way to do it. I suppose this will come sooner or later.)

Lag: This was indeed a problem early on with the 5X. I'd previously reboot every morning. Now, the monthly security updates are pretty much it for my rebooting. That said, every once in a while, background tasks do drag the phone down. Misbehaving apps that don't know how to play nice (which seems to include TiVo, AT&T UVerse Voicemail, and sometimes others) can drag down the whole phone.