Ok a kind member on totalwar.com has given me a heads up on how to stop the lag when scrolling across the map in campaign games:

In search bar type: "Power & Sleep settings"

1. Open Power & Sleep
2. Additional Power Settings
3. Change Plan Settings
4. Change Power Advanced Settings
5. Set to "High Performance" in drop down box

That fixed the issue for me and I use Windows 10 Operating system.