You can copy-paste the URL (e.g. from YouTube) into VLC (the keyboard shortcut is `Ctrl`-`n`).
Then you can adjust the speed in VLC with _Playback→Speed_ or click on the `1.00x` icon at the bottom right, then you get a nice slider. The keyboard shortcuts are `-` for slower and `+` for faster (the latter one is not working on my system for some reason).