Thanks @morti .
huyvzhope if you are using chrome (for sure on windows, idk about other OSs), right click on the playing video, click "Show all controls", then you need to click the 3 dots in the bottom right of the video (I usually put the video fullscreen, only then I can reach the 3 dots without clicking on the lookmovie status bar) and in "Playback speed" you can select what you want. Then right click again on the video and deselect "Show all controls"