How Fast is Real-Time? Human Perception and Technology

6 min readMichael Carroll on Nov 13, 2022

In 1968 Robert Miller published his classic paper Response time in man-computer conversational transactions in which he described three different orders of magnitude of computer mainframe responsiveness:

  1. A response time of 100ms is perceived as instantaneous.

  2. Response times of 1 second or less are fast enough for users to feel they are interacting freely with the information.

  3. Response times greater than 10 seconds completely lose the user’s attention.

From this Miller concluded that a consistent 2-second response would be ideal. Years later this same value of 2 seconds has been used as a performance target for web-based applications. Today’s real-time applications, however, require near-instantaneous responsiveness. Does even 100ms cut it?  The answer depends on the context.

How Fast Can a Human Process Input?

As humans beings, we have the curious inborn ability to observe and experience the persistent passage of time. The architecture of our human brains, however, limits our sensory perception in a way that prevents us from reacting to our perceptions within a certain short timeframe. This timeframe is commonly known as Reaction Time.

Human Reaction Time

The average human reaction time is on the order of a quarter of a second (250 milliseconds). Don’t believe it? You can test your own reaction time with this little test.

As you know, some humans have better reaction times than others. Fighter pilots, Formula One drivers, and championship video game players fall into the 100 – 120ms bucket on the left side of the curve.

How much of that time is spent receiving data versus mentally processing and physically reacting?

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Real-Time Latency: From Eye to Brain

Reaction time is a complex subject and includes several different components of mental processing including:

To really understand how fast real-time is to the human brain we’ll focus on the Sensory Perception phase. This is where our senses receive the incoming data from the outside world whether that be visual or auditory.

For example, the time that an image of a tiger arriving on your retina takes to travel down your optic nerve into the visual cortex is incredibly fast. New studies show that humans can interpret visual cues seen for as little as 13 ms (about 1 in 75 frames per second).

As the brain receives the incoming data stream, an asynchronous process acknowledges the input and admits it into our consciousness. Now aware of the incoming data stream, another part of the brain applies context to the stream so that a decision can be made about how to react. All this happens very quickly. (Cats are nearly twice as fast.)

How Does Unwanted Latency Impact Human Performance?

While there is more involved in human reaction time than just mental processing, the important concepts here are:

1. The fastest rate at which humans appear to be able to process incoming visual stimuli is about 13 ms. Receiving a stream of data faster than this will only underscore the limits of our perception.

2. Increasing latency above 13 ms has an increasingly negative impact on human performance for a given task. While imperceptible at first, added latency continues to degrade a human’s processing ability until approaching 75 to 100 ms. Here we become very conscious that input has become too slow and we must rely on adapting to conditions by anticipating input rather than simply reacting to input.

In a duel, for example, a 100 ms lag matters. Especially if it is random and cannot be anticipated.

Implications for Real-Time Application Developers

Real-time applications have varying tolerances to data stream latency. Typically those applications with very demanding targets include:

It is these types of applications in which real-time human perception and interaction are required. Given the resources required to build and maintain a real-time data stream network to support these types of applications, many developers make the strategic decision to outsource the messaging layer in order to focus more on the application itself.

Online Gaming

While turn-based games, role-playing and strategy games typically do not rely on real-time movements or actions and they can tolerate latencies of up to 500ms or more, for Massive Multiplayer Online Gaming (MMOG), real-time is a requirement.

As online gaming matures, players flock to games with more immersive and lifelike experiences. To satisfy this demand, developers now need to produce games with very realistic environments that have very strict data stream latency requirements:

A delay of even 100 ms reduces player performance in Twitch games by a measurable amount. It becomes noticeably difficult to track targets effectively and forces players into predicting movements.

Overall game enjoyment continues to decrease as latency increases and players experience jerky playback, ghosting, and out-of-sync behavior that ultimately ruin the game for all players involved.

Given these parameters, to be successful MMOG architecture must consider network performance as a fundamental requirement to ensure Quality of Experience for gamers. This architecture needs to be capable of delivering thousands of simultaneous data streams with latencies as low as 50 ms or better and to make it even more challenging, it must do so at scale for players in different geographic regions, on different access networks, using a range of devices.

“PubNub allows us to focus on our application, rather than the backbone network that supports it and the worries that accompany that. Knowing that we don’t have to setup a whole monitoring system to make sure our backbone network is running and sending messages is amazing; no crashing, no hardware reboots, and no worries,” said James Ross, co-founder and Operations Manager of NodeCraft Hosting.

Bidding & Auctions

With any auction, every split second counts. Success depends on making all bidders feel there’s seamless, reliable, real-time engagement for everyone, wherever in the world they may be. Having the right end-to-end platform for delivering real-time capabilities like real-time notifications, alerts, and updates is an imperative part of the user experience.

A reliable and scalable platform solution is the key to online auction success because:

Real-Time Collaboration

Another interesting example of the use of real-time data is in the area of e-learning and collaboration. In the online classroom, it’s essential to have reliable real-time communication between devices. When dealing with a classroom full of students with short attention spans, devices need to be able to signal between each other as quickly as possible. Otherwise, you’re losing the attention of the entire class. However real-time data stream network design is an outside core competency for most.

“We didn’t know whether we should hire people to do that, whether we’d have to increase the size of the team, and eventually we just sat down and thought, ‘this isn’t what the core of our business is about. We shouldn’t really be spending loads of money and time trying to make real-time work, when we should be focusing on our own business challenges. Real-time was a requirement for our business, but not a business challenge that we should have to solve,” said Liam Don, Co-founder and CTO of ClassDojo.

Live Entertainment

The hallmark of a successful virtual event is if the audience is engaged with not only the performers but also each other. But just because virtual events have been part of our reality for a few years now, doesn’t mean that every event organizer (or platform) provides the necessary features to allow for engaging, interactive events.

A few key features that developers should keep in mind when building apps for virtual events are:

PubNub and Real-Time

Our real-time data API is flexible, so you can roll out these features quickly. Our globally available messaging infrastructure then lets you expand as your app, and needs, evolve. Getting started is easy. And, if you have any questions, we offer support 24/7 worldwide. With PubNub, you can start building