How to set S3 only save user upload files

Yes, I am using cdn for S3.

But I still wondering, if
does discourse server(asia) → cdn → users(asia) faster than S3(US) → cdn → users(asia)?

I saw there is a parameter named DISCOURSE_S3_UPLOAD_BUCKET, it looks like a solution to me but after I replace DISCOURSE_S3_BUCKET by it. I cannot rebuild my discourse