I have just made this thread for those who don't know about this solution. I have tested this on my Mac as well as my PC; both have had reduced lag in-game because of this solution which I've found.
Step 1: Open up Blockland of course
Step 2: When Blockland is finishing loading up, click "options"
Step 3: Uncheck "fullscreen" (might work in fullscreen too, haven't tried)
Step 4: Check "borderless"
Step 5: Click "apply"
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If this doesn't work, I highly suggest changing the resolution in "display settings" to 800 x 600, and then do the other steps listed above.
For even LESS lag: turn off shaders to minimum or none, and turn off particle quality to either medium-minimum
I also don't know if I placed this into the right section; because in Help I thought that you only made topics ASKING for help, not tips on how to help.
I hope this helped!