Spectators don't cause lag. What causes lag is the server having to receive data from 100s of PLAYING characters, convert that into movement/actions, and then calculate the physics, damage, position, etc. of everything in the game.
Spectators don't send info to server, they receive from it. The only possible way spectators affect lag is from the server having to send the info to the spectators, but this is unnoticeable. For instance, if you type something into chat, that sends extra info to hundreds of players. Does the game grind to a halt every time someone says "hey"? No, it doesn't. In fact, tons of people can talk all at once without any affect on performance. So how could sending info to spectators hurt? IT DOESN'T.