I dont have a really powerful operation system so is there any way through which I could decrease the polycount?
I have already removed doubles from my meshes and un subdivided.
You can use a Decimate modifier, setting it in “unsubdivide” mode and slightly increase the iterations (1 is normally fine). This will drastically reduce the poly count of your mesh, but at the same time it won’t be noticeable! As the quality of the model remains almost intact (that is, if you don’t exaggerate with the iterations, of course)!
Simplify
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