Programmatic
Latency
Latency is the time delay between a request and a response; in advertising, it is how long auctions, ad servers and creatives take to return and display an ad.
Latency is waiting time, measured in milliseconds. Every step of ad delivery adds some: the ad request, the header bidding auction, each SSP calling its DSPs, the publisher ad server decision, loading the creative and running tracking scripts.
High latency hurts everyone. Pages load slowly and users leave, ads appear after users have scrolled past (hurting viewability), and bids arrive after the auction auction timeout and are thrown away. Mobile networks in emerging markets make latency particularly costly. For this reason, server-side header bidding, lazy loading and lightweight creatives are common optimisations.
Buyers also operate under strict latency limits: in OpenRTB the "tmax" field tells bidders how many milliseconds they have. That puts pressure on DSPs and pre-bid filtering vendors to make decisions fast. Unusually uniform latency patterns can also help spot automated traffic.
Think of it like this
Latency is like the gap between ordering at a counter and getting your coffee: a short wait is fine, but too long and people walk out.
An example
A publisher's header bidding adds 800 ms to ad load time. After moving five bidders server-side, it falls to 350 ms and viewability rises from 58% to 64%.
Related terms
Auction timeout
An auction timeout is the maximum time an auction waits for bids; responses that arrive after it are ignored, even if they would have won.
Header bidding
Header bidding is a technique where a publisher offers each ad impression to many SSPs and exchanges simultaneously before calling its ad server, so all buyers compete at once.
Server-side header bidding
Server-side header bidding runs the header bidding auction on a remote server rather than in the user's browser or app, reducing page weight and latency while connecting to more bidders.
Lazy loading
Lazy loading is a technique in which ads (or images) are requested and loaded only when the user scrolls near them, rather than all at once when the page first opens.
Viewability
Viewability measures whether an ad actually had a chance to be seen on screen; the MRC standard is 50% of pixels in view for one second (display) or two seconds (video).