If you sell your Bitcoin for price x then obviously you will still have x when the price is y. You will only have more money in value if you buy x bitcoin and then the price of Bitcoin moves from x to y, so now even if you have x amount of bitcoins but their value will be y.
And, it doesn't take time for trading in exchange, because once you have deposited the money in an exchange, then you don't need to wait for any confirmations for doing any trade within that exchange, unless you want to withdraw your trade every time which is not what people usually do. You deposit in the exchange, and then you trade bitcoins there for whatever time you want to, and then you withdraw your funds when you don't want to trade anymore.