I haven't experienced any lag issues at all and the game is excellent and massive. Incredible amount of content, places and people and the difficulty setting can be changed at any time, and changed back, if you're struggling. The main storyline is of perfect length and there's an almost uncountable number of side quests and missions.
The only thing holding it back from 5 stars is the repetitiveness of some side quests. You can only clear out so many dungeons of similar format before it gets tiresome. The game is essentially a (less-buggy) Fallout 3 with a Middle-Earth-esque makeover. I personally prefer Fallout because of the theme but that's not Bethesda's problem. Also, I found Demon's and Dark Souls more playable due to their unchangeable difficulty.
These are relatively minor, and preferential, issues and if I could award an extra half star it would definitely get it.