First of all, you should disable the guide option in the setting. That can save up to 1min just in the first mission.

Also, if you manage to met Paddock at the AA-battery without Doyle by your side, the conversation between the two will be cut short. It saves approximately 12-13 seconds. You can manage to do that by ordering Doyle to stay behind.

Near the end of the mission, when you killed the MG at the back of the road, the game guide you with the compass to use it to kill the other waves of german but don't use it. Instead ask your unit to go where the germans spawn and go where Cassidy will be at the end of the mission to complete it faster. That saves ~10sec.