Las Vegas has a pretty nice airport. And it's one of the busiest in the country. Overall, though, it is usually not a place that you are going to get stuck with long lines at check-in or at the TSA checkpoints. Even if it looks like a long line, it will likely move pretty quickly. That being said, I would generally take the advice of most experts and try to arrive at least 2 hours before your flight leaves. That will be doubly important if you are flying out at a particularly busy time like Sunday afternoon or evening as that is when the weekend crowd will be heading home. Even if you've heard that your flight might be delayed, unless the weather conditions are pretty dire, you might still want to arrive a few hours before your scheduled departure, as a predicted delay may not come to pass.
Of course, there are plenty of distractions to keep you amused should you find yourself waiting on your departure. Yes...you can actually play slots at the airport. They are everywhere. Now, I've heard that these particular machines don't pay off so well and I'm inclined to believe it, but it is something to do. I usually plan to eat something before I leave and there are quite a few places to do that too. And, if you promised someone a souvenir but totally forgot to actually get it, you can still do so before you leave.
There is actually a a pretty good little exhibit of sorts on the history of air travel to Las Vegas on display in Terminal 1 and you can easily kill 10 or 15 minutes looking at the displays. If your portable electronic device is starting to die on you, there are charging stations everywhere so you can plug in and recharge while you entertain yourself on your iPhone or Kindle as well.
I'll just say one more thing about the fabulous McCarran International Airport. The people watching can be spectacular. Just wander around and watch the different expressions on people's faces who are arriving and those that are departing. And, if you're really lucky, you'll be able to catch all the strippers flying into town Friday night.