Why would performance decrease as the number of posts increases? Does the plugin re-evaluate every post at each additional load? Or is it parsing the generated HTML each time?

It's only when you check/uncheck the box, so I'm not sure if it's really avoidable. In order to hide/show resteems, it has to move all the other posts up or down. When I first encountered the problem, I was on the verge of crashing my browser when I tried to toggle "show resteems" on. Even 200 posts takes 2-3 seconds to rearrange things with my configuration. Tolerable, but slower than I'd like.