[davidrusu]

There's a few of these plugins.

What I actually want is a plugin that makes the time wasting sites unbearably slow. This way if I need to check something on one of these sites, I can do so without disabling the plugin.

Maybe you can do something with increasing latency the longer you stay on the site.

[pkinsky]

There's an extension called Delayed Gratification that just introduces a 30-second delay for the initial loading of sites for which it is enabled.

[Klathmon]

I tried this once, and funnily enough it worked for a bit, but I ended up pulling out my phone while waiting for a site to load one day, and then laughed at the irony of what was happening.

[wingerlang]

In the screenshots of this plugin there is a "this is important" button that seems to let you into the site without disabling it.