The lag is because they host all of the places on their own servers. Another bad thing about their places is that there is no search button, so you have to go through a ton of pages manually.
Actually it's because there's no client-sided predicting. What this is, basically, is where the client guesses where the player is actually at, which reduces the need for the server to say, "Okay. You. Are. HERE." This is what causes your character to jump around when there's a high latency between you and the server: you're getting an old position packet, so you jump back to where you were until you get a new one, then you jump back into place.
Blockland has this. ROBLOX does not.
It's also because ROBLOX has actual server-side physics, which can cause massive lag when 20000 bricks from the Titanic fly at, well, titanic speeds in random directions.
ROBLOX has this. Blockland does not.