Client side header bidding requires an HTTP request / response for each bidder, followed by an HTTP request to the primary ad server (very often, Google Ad Manager), that usually runs its own ad selection process (against lime items from direct sales, and programmatic demand sources such as Google Ad Exchange). Server side header bidding offsets the call to each bidder to the server side and it somewhat improves latency, but it still takes some time to complete the auction, and there's still need for an extra HTTP request / response to the header bidding server.