Here is a tip for anyone else having awful performance when they start or load a game.

1 - Click on the menu button in the top right corner.
2 - Click on the gear icon to open the in-game graphics setings.
3 - Set the shadows setting to Low then back to High.

This works4me(TM) and turns sluggish performance into good or decent. Oh, by the way this is on Windows.

I guess this is a good place to add my tip as well.

If you find you are getting artefacts flashing up on screen when zoomed in close then you should turn down the "Geometry" setting which effects the draw distance. I had this set to high, I turned it down to medium and problem solved.