Substance materials creating a huge lag at start-up in a packaged build

Hello Everyone,

I am currently in a weird scenario when using substance materials in my map. I am using Unreal Engine 4.19. In editor, the map plays well without any FPS loss. Whenever I package the project and play it, while starting up it lags to a point of freezing the screen. FPS during those freezes ranges from 0.1 to 0.5. Once all assets are loaded, there is no FPS lag after. With normal Unreal materials, there is no FPS loss what so ever, in editor or in a packaged build. I am confused as to why this is happening. If I use too many substance materials, FPS reaches 0, system crashes and I have to restart my PC. There are no errors or warnings in Output Log while packaging. Is this a substance issue or an unreal issue? I am confused. Please provide me a solution for this. I am stuck!!