If you have anything which could be causing the game to crash while initializing or running (E.g. in my game I had drones follow me, but they crashed my game) then I would suggest making their functions that are causing the crashes become coroutines with delays up to what causes the game to stop lagging/crashing.
In my experience, the common cause for this is having a infinite loop into the Awake or Start of some script. If you have any loop (for, while) check that its correctly finishing.
As Debug.Log will not show anything, you can try disabling all the objects into your scene and enabling them one by one.
If you can still not find the source of the problem, you may need to learn how to use a proper debugger.