Although I don't have much experience, GDAX offers near instant updates to its trading portal. In addition, it offers candle stick charts down to 1-minute intervals. I've also had good experience with crymptocompare API. I use it in the form of the CryptoFinance app for google sheets. By using the command =GOOGLEFINANCE("BTCUSD") in a cell, it will fetch the updated price for any coin that you can see on cryptocompare.