Poor sluggish performance after upgrading to Yosemite

I am facing very sluggish performance in my MacBook Pro after I upgraded it to Yosemite. Mine is 8GB, i7 mid 2012 system.

Usually I find my unused RAM <25MB even if I work in terminal, say around 8 Safari tabs open (excluded fb) and some light-weight programming editor open.

I remember I never faced such sluggish performance in Mavericks or Mountain Lion. I have closed all unwanted graphical effects, but still no improvement on performance.

Any suggestions?

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Below is the screenshot of my activity monitor:

Please note my Preview was closed while I took this shot; which is in a way super weird.

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CPU usage: