Okay, I can confirm this. With a small amount of posts loaded in the window, the rendering times are acceptable (40-60ms each for keyUp / keyDown)
However, as more posts are loaded into the window, the rendering time of the widget gradually increases:
50 posts: ~50ms
100 posts: ~70ms
150 posts: ~100ms
200 posts: ~130ms
etc.
There’s certainly a scaling issue here, but it doesn’t really manifest itself except that it’s rerendering on keyUp and keyDown 
(I need both for separate events), meaning we get to that 300ms limit of human perception quite quickly.