This sets the speed to 1.125.
But any rate will do: 0.69, 4.20, 6.666.
Doesn't matter.
It's kinda obvious that it will throw an error if there is not at least one element on the page with the classnames 'video-stream html5-main-video', but nevermind.
I then added this snippet into a browser-bookmark i can click anytime i'm watching a yt video and want to set the speed.
If you replace the speed number with Number(prompt(“New speed:”, 1.15)) you should get a dialog box to put a new speed in, so you don’t have to make multiple bookmarks.