I pass through McCarran a few times every year. The security lines are never too bad (about 10 minute wait), but I'm sure they're awful at peak times (mornings, Sundays).
So why two stars? I stopped a Burger King past security and ordered a medium-sized cup for their soda fountain located on the side. For $3, I was pretty shocked when I saw NO REFILLS taped above the fountain. This didn't stop some people from refilling, but when one of the employees saw, she actually enforced it!