Some video streaming websites like YouTube provides an option for speeding up/slowing down videos; some don’t. The trick is simple, find out the Video object via

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document.querySelector("video")

and then set its playbackRate property to the desired value

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document.querySelector("video").playbackRate = 2.0;   // For 2X speed-up

These changes have to be made in the Developer console in Chrome or Browser console in Firefox.