Why missions takes so long to finish even on easiest difficulty

I am playing the campaign in order and on average I played about 2-3h per mission but the the Mongol mission number 3 (into the china) took me 5h!!! Of course I reloaded many times cause I lose the time race against the wonder built by enemy. What am I doing wrong? I play this game on standard difficulty and its a chore to play against AI. Hiw can human possibly control so many things at a time. Is there a lower difficulty mod? Cause I like the game concept and the graphics are nice but the gameplay makes me anxious. I just want to play thus on leasure speed similar to Anno or Civilization (although its turn based so its even easier). I am actually struggling on battles. The AI is crushing my army wso I have to rebuild it like 10 times a mission to complete it. Do you have some tips on combat to make it easier for me?