In Diplomacy, how does a time limit per round affect overall gameplay?
The diplomacy phase is the significant part of the game. So limiting this will affect the game. This site advices a longer period:


  
  Combinations and agreements among the players may affect the course of the game a great deal. These are determined during the diplomacy
  period which takes place before each move. This period lasts 30
  minutes before the first move and 15 minutes before each move
  thereafter. These periods may end sooner if all players agree at the
  time.
  


So 10 minute is slightly shorter. This is no problem for experienced players. But with inexperienced players it is possible too short. Not everybody will have the time to meet with everybody. People will miss out on some opportunities and they possibly get frustrated. 

My advice is to limit the diplomacy phase, for the first year to 20 minutes and add another 5 minute to write down the orders. For the next year, you can go to 15 or 10 plus another 5 top write down the orders. This will create a more friendly game for new players.