Will polymorphic relation cause slowness on big data?

Good Evening/Morning,

I am developing a social media type database which will have millions of users, I have not worked with a data that big so I am a bit concerned about the approach I am taking with the comments part. There will be comments on different entities such as Posts, Pages, Groups, and 3 to 4 more. So what I am asking is that if anyone out there have implemented polymorphic relation on big data can you share your experience on its performance on big data?

via Chebli Mohamed