Placed blocks rarely cause lag, most lag in minecraft (par the occasional lag spike caused by chunk loading) is usually to do with either animations, or entities.
I'd recommend installing Optifine as it will help combat some of the lag minecraft causes, and can give you generally high FPS if you have a decent computer.
If that doesn't help, then sadly you're going to have to kill some mobs, as by the sounds of the posts above me they're causing the issue.
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