That's really a problem... it's one of the best discussion boards, but the responsive support is missing and makes it "unusable". Speed is okay and great, but font-sizes and the flow of the threaded elements and so on... mmmrrrr. :(

I've created a screenshot of my phone with a standard smartphone-browser and it looks like that: http://muli.cc/i/c4f86.png

You can tweak some stuff with the code below in the header, but there are many more problems - also navigation-buttons and so on.

 
<meta name="viewport" content="width=device-width, initial-scale=1" />
 

You can use the linear-modus, but only, if there aren't too many replies (padding left).