What is an ad exchange and what happens in under 100 milliseconds

Key takeaways

• An ad exchange is a real-time marketplace where publishers auction ad impressions to advertisers.
• Real-time bidding (RTB) connects a publisher's supply-side platform (SSP) to advertisers' demand-side platforms (DSPs) one impression at a time.
• Ad exchanges differ from ad networks, because exchanges run open, per-impression auctions, while networks bundle and resell inventory at negotiated rates.
• Hitting that 100-millisecond window consistently depends as much on the server infrastructure underneath the auction as on the ad tech stack running on top of it.

What is an ad exchange?

An ad exchange is a digital marketplace where publishers and advertisers trade ad space through automated auctions that run in real time. Every time someone loads a page or opens an app, the exchange runs a live auction for that specific ad slot, and the highest qualifying bid wins before the content finishes rendering.

Google Ad Manager (formerly AdX), PubMatic, and Magnite are the largest exchanges running these auctions. Each one connects thousands of publishers on the supply side to thousands of advertisers on the demand side, and none of them communicate with a human during the transaction. The entire negotiation happens in code, on a clock measured in milliseconds.

Speed determines whether an ad exchange works at all.

How ad exchanges work: the anatomy of a 100ms auction

Real-time bidding follows the same sequence millions of times a second, across every exchange:

1. A person loads a page or app, and the publisher's ad tag fires a bid request.
2. The publisher's SSP packages that request with context: ad slot size, page category, user data available for targeting, and a floor price.
3. The exchange broadcasts the request to every connected DSP bidding on that inventory.
4. DSPs evaluate the opportunity and respond, each running its own targeting logic to decide how much the impression is worth to an advertiser.
5. The exchange picks the winning bid, clears the auction, and returns the winning creative.
6. The ad renders before the rest of the page has finished loading.

All six steps happen in about 100 milliseconds. To put that in perspective, a single human blink takes 100 to 400 milliseconds. Here's roughly where that time goes:

The IAB Tech Lab's OpenRTB specification sets the technical protocol that most exchanges and DSPs use to communicate these bid requests, and it builds in strict timeout windows.

Ad exchange vs. ad network

People use ad exchange and ad network interchangeably, but they describe two different models.

An ad exchange is an open marketplace that runs live, per-impression auctions. An ad network acts as a middleman: it aggregates inventory from multiple publishers, bundles it, and resells it to advertisers, often at a fixed or negotiated rate instead of a real-time bid.

Neither model is inherently better. Ad networks make sense for advertisers who want simplicity over granular targeting, or for publishers with inventory too small to attract exchange-level bidder competition. Exchanges make sense when advertisers or publishers want pricing transparency and real-time control.

3 types of ad exchanges

Different exchanges run different kinds of auctions that fall into one of three categories.

1. Open exchange

An open exchange is a public marketplace, open to any advertiser or publisher who connects to it. It offers the broadest reach on both sides, but publishers give up some control over who bids, and advertisers compete against a large, unfiltered pool.

2. Private marketplace (PMP)

A private marketplace (PMP) is an invite-only auction. Publishers select which advertisers can bid, which gives them more control over pricing floors, ad quality, and who sees their audience data.

Advertisers get access to premium, vetted inventory, though the pool is smaller.

3. Preferred deals and programmatic guaranteed

In preferred deals and programmatic guarantees, the publisher and advertiser negotiate a fixed price directly, then execute the transaction programmatically.

Preferred deals give advertisers first look at inventory before it hits the open market, with no purchase commitment. Programmatic guaranteed adds a committed volume on both sides, combining the predictability of a traditional ad buy with the automation of programmatic delivery.

Why advertisers and publishers use ad exchanges

For advertisers:

• Precise targeting, since DSPs can evaluate user and context data before every bid.
• Pricing transparency, because bid data shows what an impression costs, rather than what a rate card says it costs.
• Reach across a large number of publishers through a single connection point.
• Brand safety controls, including the ability to exclude specific sites, categories, or placements.

For publishers:

• Competitive pricing, because multiple advertisers bid against each other for the same impression instead of accepting a single negotiated rate.
• Higher fill rates, since exchanges connect a publisher's inventory to more potential buyers than direct sales.
• Control over floor prices and over which advertisers can bid.

In 2024, over 88.2% of US digital display ad spend transacted programmatically, according to eMarketer. Ad exchanges, running real-time bidding underneath that spend, process a share of every ad impression served online.

What can go wrong in an open auction

Real-time bidding solves a lot of problems, but it introduces its own risks too. Most of them trace back to how little time an auction actually gives anyone to react.

Take timing, for example. Every DSP in the auction gets a limited window to respond, and if a bid doesn't arrive in time, the exchange drops it and moves on, no matter how strong that bid would have been.

That same openness creates a second problem: because open exchanges accept traffic from a broad range of sources by design, they also attract invalid traffic and spoofed inventory. That’s exactly why brand safety tools and supply path optimization have become standard parts of the stack rather than optional add-ons.

Publishers pay a price for that openness too. Selling on the open market means giving up some say over which advertisers see their inventory. That’s a big part of why private marketplaces and preferred deals exist in the first place.

Run those three threads together and one pressure point stands out above the rest. Fraud protection and deal structure can manage a lot of risk, but neither one can buy back a bid that showed up too late.

What it takes to run bidding this fast

Every millisecond spent on an unnecessary network hop, a delayed response from a shared server, or an inconsistent compute environment is a bid that doesn't make it back in time. At the scale ad exchanges operate, that's the difference between an advertiser winning inventory or losing it to a competitor whose infrastructure responded faster.

And that is why the infrastructure decision is important. Hyperscale cloud gives you flexibility to scale, but its shared, multi-tenant infrastructure introduces variability too. A noisy neighbor can add latency at exactly the moment your DSP or exchange infrastructure needs to respond.

Traditional bare metal solves the noisy neighbor problem with dedicated hardware, but its slower provisioning makes it hard to scale with campaign volume.

The answer for an increasing number of adtech teams is a hybrid approach. If your ad exchange or DSP infrastructure is losing bids to timeouts, or if your current environment can't tell you whether the problem is code or compute, speak to a specialist about adtech infrastructure to see where the milliseconds are going.