I really liked this extension, and it worked great to stop me from visiting distracting sites. However, it looks like a recent Firefox update has broken it. I jumped into a debug window and I think that the problem is that Firefox now has a "loading" tab change sent before it actually sends the updated site. Slow Down seems to only look at the "loading" tab, not the actual site.

Does not abuse processing power compared to some extensions of the same category (Leechblock).

I don't usually write reviews, but I have to say that this addon is almost perfect.
Works like a charm and is privacy friendly!

Does the job, although it might take a few tries to enter the URL in the right format. I picked this one because it didn't require the "all web pages" permission that the other extensions of this type did - so I'm much more confident of my privacy.