Lags will occur if you have greater than 600 ally in the map and if the buff is highly particle. but it will not cause a lag if the buff is just normal like the buff of Brilliance aura or devotion aura.
EDIT:
@Hell_Master
Why lag?? I tested it in the map with brilliance aura global.
Since even in DotA ( the global aura of Crystal Maiden ( Brilliance Aura ) )