Why doesn't Minecraft run smoothly?

My Laptop has 6 Gb of RAM, an Intel Core i5-3317U, an NVIDIA Geforce GT 635M 2Gb graphics Card and Windows 64bit. I also have Java 8 Update 45 Installed. Minecraft lags extremely bad on both Servers and offline, both modded and not modded. I even turned to the lowest Settings, but now Whenever I break a block, it reappears and I have to break it again. Please Help!

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