Google is releasing a new video calling app later this summer. It's called Duo, and it's probably the fastest and simplest video calling app I've ever seen. Like its companion chat app, Allo, Duo is mobile-only. It uses your phone number as an identifier, and it will work on both iOS and Android.

The interface for Duo is so simple and sparse it almost seems silly to describe it, but here goes. When you open it, you're greeted with a selfie-cam video preview of yourself. Check your hair, then tap one of the giant circled photos below to start a call — or scroll up on the panel to see more contacts. You call, the person on the other end of the line answers, and you're having a video chat. You can mute your microphone, you can flip the camera. You can't do a video conference, though.

And that's really it. It took all of five minutes for Google to show me the app last week. When it was over, Erik Kay, director of engineering, communications products, couldn't help but point that out: "You noticed the demo was super short and that's because that's all it needs to be." The whole app on Android weighs in at around five megabytes.

So don't let the lack of bells and whistles fool you, because the engineering team at Google spent its time on something more important: making Duo crazy fast. The premiere feature, available only on Android, is that when you are receiving a call you see a live video of the person calling you right on your lock screen. When you answer, the video call is already going — instantly.

Google also spent a lot of resources under the hood. Google says it has spent lots of time optimizing for speed and latency — it should work even on relatively flakey connections. Duo dynamically adjusts the video and audio quality to suit whatever connection you're on. But it also has one more trick — it's able to seamlessly handle your phone switching from Wi-Fi to cellular and back again without dropping the call.

Google has lots of nerdy, technical things it can tell you about how it achieved all this. It optimized webRTC and connects the calls using QUIC over UDP for a much faster initial end-to-end encrypted connection.

The easy thing to say is that Duo is taking on Apple's FaceTime. Duo is faster and it's cross-platform. But it's also mobile-only and it's one more app you'll need to convince your friends and family to install.

Amit Fulay, group product manager of communications products, told me that "our team's motto when we started the project was: performance is the feature, video is the UI." And even though that might sound like corny Silicon Valley buzzword speak, from what I saw of Duo, it was true. Compared to Skype or Google's own Hangouts, Duo is a breath of fresh air.

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