It's been a lifesaver in tackling my internet addiction. It has the useful 'delay option' and other nifty features that helped wean me off social media sites.
Personally, I have it installed on all my desktop browsers and Firefox on Android. I give myself half an hour quota per day for the time waste websites (Youtube, Twitter, FB, Insta, etc) after which a 60 second delay rule kicks in. I've found that my monkey brain almost never bothers waiting for a minute for the page to load unless it's absolutely necessary.
One goal is to quit something completely. For that, you probably want delay across the board so that it eventually saps the joy out of the experience.
Another goal is practicing moderation. With social media as an example, maybe you want to stay up to date on something, so 20 minutes a day is fine, but you don't want to fall into the trap of killing 3 hours on there. So, you could make it full speed for 20 minutes of daily usage, then gradually degrading from there. By the time the speed degrades, you've probably done all the truly appealing parts anyway and it's easier to walk away when nudged.
Artificially slowing down those time waster websites instead might do the trick. Not so slow it becomes unusable (or I'll turn it off) , but slow enough that over time I start associating it with a bad experience.