Biggest issues with laptops on ultra settings is heat. Things really get cooking so you really have to ensure the vents stay clean
Lag? don't matter much really. So places you'll be smoking on ultra settings, others, at a crawl. I do alot of wedding photography/video, and my desktop in a Intel i7, 16g RM and a Nvidia GTX970 4gig card. "usualy' at ultra on a wedding 12-16fps, some higher, sometimes down in the single digits. Faster than my laptop I used to use for sure, but also runs vastly cooler