While the UI of YouTube only shows only limited set of high-speed options, it is possible to set the speed to any floating point value. Even better, one can do so from their favorites bar with bookmarklets.
Simply add any of the following code snippets as bookmarks.
If you have a fixed speed in mind, e.g. 2.5:
Or save this line to show a prompt that asks for a floating-point input:
Which results in the following prompt:
Works with YouTube and Vimeo.
The speed display in the video player will remain to show the last setting.