Basically it's jQuery mobile version of reddit, and it's super fast compared to whatever framework they are using for the regular mobile site.

I just want to read posts, comments and answer them, nothing more, the nag screens telling me to download an app? the loading screens or whatever, just not... why is that i.reddit version so fast and the regular one so slow?

This was the old mobile website, not i.redd.it.

Forcing people towards their mobile app