Well, I lag at around 11k bricks + no frigging idea why, I can play much more "graphic eating" games than Blockland with no lag at all.
But when I go into servers with over 11k bricks, yes I lag, but only when looking at a lot of bricks at the same time, for example, if I look at 25 thousand bricks, I get 1 FPS (no joke) but when I turn around and I face nothing or maybe 2 thousand bricks, my FPS rises back up.
So I guess it doesn't render things not in view?