I was going to comment about this.

The balance you see in Coinbase, even with "instant buy", functions more like "I owe you's" depending on how much funds they have in their reserves. You won't have 100% of the funds you bought to move immediately.

It does take 7-10 days for funds to arrive in Gdax. The difference is that you own the full liquidity the moment you make that trade.

I personally use Binance instead of Bittrex, but the idea is similar nonetheless.