You were right. It was the other windows in the background that was causing it.
Thank you for the reply to my silly question and sorry for the delay in my response.

I have encountered a problem while playing the game. Every 10 to 15 minutes there is a noticeable frame rate drop.
It lasts for about 1 second then plays normally. I have tried to start a new game and the issue remains.
I have included some screen caps of the issue before, during and after ...