No way. You only pay the FX spread when you transfer your funds to USD and when you transfer them back.
You can trade as much (or as little) as you want and you can leave your funds in USD for as long as you like - it’s your money remember!
Other brokers around the world slug you wit an FX commission for every trade by forcing you to convert back into your local currency every time you trade. We don't think you should settle for that.