I sincerely hope you aren't using "No" as a blanket response to what I just said. Blockland will slow down in terms of frames per second when you tab to another program. Well, do this. Compare your RAM / CPU usage when playing someone else's server to RAM / CPU usage when you're hosting your own. If you're using more while hosting, then logic would suggest that it's to be expected that returning from a tab-out might take longer.