Why Play Mode is taking a long time on Pro?

Only on first time you enter Play Mode after you launched the Editor. And only on 4.6 Pro / Unity 5, 4.6 Free don’t have this problem.
The Play Mode usually reads something from my HDD then put it to memory, eating memory resources and harddisk IO. The process takes about 10-15 minutes before the Play Mode actually work. Any Play Mode after that only take few secs.

My project size is around 2 GB, the 1st time Play Mode usually takes the memory around that (2GB) then free them, going back to 800MB or so. I don’t use SSD.

I think I found perhaps the answer. All of a sudden I ran into a situation where when I would hit the Play button to launch my Unity project, I would get an incredible delay that would last 1-3 min.

I tried tons of things (rebooting, reloading of unity, code changes, undoing any of my changes to prefabs etc)

Solution:

Delete all your GI Light Cache files.

%AppData%\LocalLow\Unity\Caches\GiTemp

Hope this helps you out.