This topic misses the forest for the trees. Even if there is a cognitive difference in people who speak multiple languages, the magnitude of the advantage/disadvantage pales in comparison to the far more impactful difference in the understanding of the world that proficiency in another language gives you.
If you're a monolingual American born English speaker and you learn Chinese and go live in China for a year, you're going to come back with a much different understanding of Chinese culture and its people. Who cares if your IQ went up or down half a point on account of learning to speak mandarin- your understanding of the lives of billions of people just increased dramatically. That's why it's good to learn a language- culture, not cognition.
If you're a monolingual American born English speaker and you learn Chinese and go live in China for a year, you're going to come back with a much different understanding of Chinese culture and its people. Who cares if your IQ went up or down half a point on account of learning to speak mandarin- your understanding of the lives of billions of people just increased dramatically. That's why it's good to learn a language- culture, not cognition.