This audit checks whether or not header bidding requests are being made sequentially. In most cases, these requests are not dependent on one another and can be made in parallel to speed up ad loading.
Issue header bidding requests in parallel, rather than sequentially. This means that you should send all bid requests at the same time instead of issuing one request at time. If you're using a header bidding library, the library documentation may have recommendations on how to do this.
The list of supported ad exchanges and supply side platforms this audit evaluates can be found in our GitHub repository.