The code in tracking scripts varies between "short and elegant" to "massive and slow code". There is also a piece of json where the tracker thinks it is necessary to call 10 other trackers: See for example http://ib.adnxs.com/async_usersync?cbfn=AN_async_load : AN_async_load([ {"url":"http://cm.g.doubleclick.net/pixel?google_nid=appnexus&google..., "tagtype":"img"}, {"url":"https://ib.adnxs.com/getuid?https%3A%2F%2Fcm.g.doubleclick.n..., "tagtype":"img"}, {"url":"http://c.bing.com/c.gif?anx_uid=xxxx&Red3=MSAN_pd", "tagtype":"iframe"}, {"url":"http://odr.mookie1.com/t/v2/sync?tagid=V2_4265&src.visitorId..., "tagtype":"img"}, {"url":"http://match.adsrvr.org/track/cmf/generic?ttd_pid=appnexus&t..., "tagtype":"img"}, {"url":"http://pixel.rubiconproject.com/tap.php?v=4894&nid=1986&put=..., "tagtype":"img"}, {"url":"http://sync.mathtag.com/sync/img?mt_exid=13&mt_exuid=xxxx&re..., "tagtype":"img"} , {"url":"http://p.rfihub.com/cm?in=1&pub=345&userid=xxxx", "tagtype":"img"}, {"url":"http://t.mookie1.com/rsp?dnv=[TIMESTAMP]&rurl=//ib.adnxs.com..., "tagtype":"img"}, {"url":"http://t.wayfair.com/a/vendor_sync/user?vendor_id=1&uid=xxxx..., "tagtype":"img"} ]);
The slowness that we experience is from delays caused by calling scripts that call scripts that call scripts, processing large pieces of javascript and network latencies. edit: and server latencies.