Entering play mode takes too long time

Entering play mode at 1st time after opening unity it took 6min:02sec
Without any changes again when i hit play button for 2nd time it took 3min:17sec
Without any changes, 3rd time when i hit play button it took 2min:17sec
4th time - 1min:26sec, 5th time - 1min:08sec
And then on 6th click time increased to 1min:57sec. Rest of click i.e 6,7,8 the time remained same with minor difference.
I am not able to figure out what is causing this issue. Is it the polygon count? Or is it the number of object in my scene? Or is it collider? Or is it shader? Please note my scene have more then 1000 object. My game run smoothly at 35 fps with minor lag