Why is minecraft 1.7.3 lagging on my gaming pc?

Ever since minecraft 1.7 minecraft has been really laggy on my gaming pc it used to run at a steady 120 frames per second now it runs between 30-70 frames per second my Imac runs minecraft at 300 frames per second (which is above average for the mac) and my MacBook air runs at 70 frames per second (which is normal) my specs are Processer Intel Core i3 Graphics NVIDIA GeForce GTX 660 Ti 8 gigs of ram shouldn't minecraft run fine?

Added (1). Btw I have windows 7

Here are some things you can do in the video settings…

PARTICLES: MINIMAL
RENDER DISTANCE: TWO CHUNKS
VSYNC: OFF
SMOOTH LIGHTINGS: OFF
MAX FRAMERATE: SEVENTY
CLOUDS: OFF

This should give best results of lag.

HOPE THIS HELPS!
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