Based on a quick look, the biggest features it has that browser bookmark managers generally don’t have is the ability to search within saved webpages and documents without opening them. Plus, you can share bookmark collections with other people. Sounds a bit like a modern rendition of del.icio.us
Does YouTube Playback Speed Control work on other websites? I use Video Speed Controller, and it works on almost all video players. It’s got shortcuts for your preferred speed, as well as for increasing or decreasing the playback speed.
I mostly just use NewPipe SponsorBlock on Android though, which lets you set a playback speed and it’ll play all videos at that speed by default