Once historical data is loaded in NinjaTrader, it is saved automatically. I assume you did not delete historical data between testing the two broker connections. So when you connected to TD Ameritrade, NinjaTrader did not need to load the same amount as most of the requested data was present in NinjaTrader. Please see the link below under 'When does NinjaTrader load historical data?' for more information.
http://ninjatrader.com/support/helpG...rical_data.htm

If you create a chart for a similar instrument for which you did not load data before, do you experience a longer loading time than 1 second?