Are you in Fullscreen? (For you people who would flame me for saying this, 'clicking outside' for fullscreen is alt-tab. Just so you know that it's possible.)
If so, it takes a while for Blockland to re-render. Just wait, there's nothing to be done (besides switching to windowed - i did this eventually)
Otherwise, there's 5 possibilities.
1. Your computer is sh--ty and doesn't have enough RAM/other stuff. Go buy a new computer, like, NOW.
2. Background processes and other programs are cluttering up your computer's memory and stuff. Close some things, end some un-needed processes.
3. Blockland may have a problem with an add-on or a serious programming error caused by outdating/glitches. Go reinstall it, see if that helps. Then try disabling your Client add-ons and keep adding one at a time until you get the glitch.
4. Another program and Blockland are conflicting - try closing some major programs that may be doing this.
5. Your OS just might not be compatible with Blockland. If so, try another OS(Example, windows XP Pro works good for me) or just go play another game.