Is there any advantage of using in pairs instead of next?

next is faster than pairs, but only by a tiny amount. Don't use next instead of pairs for the purpose of optimizing; use what makes the most sense to you. The reason next is slightly faster than pairs is because pairs is just a call to next. This is what pairs really looks like:

function pairs(t)
    return next, t
end

Think of it this way: The atomic weight of an atom doesn't account for the mass of it's electrons; that number is just too small to mean anything. Likewise, programmers should not sacrifice more explicit code when the difference in performance is also too small to mean anything.