How does exchanges like bitrex exchange coins so fast ?
Exchanges work with promises. If you deposit 100 USD, they promise to let you withdraw 100 USD later. If you trade for e.g. BTC, they only change this promise to letting you withdraw BTC. Exchanges accomplish this by swapping promises between users.

There are no actual transactions on the blockchain (except you deposit or withdraw to or from the exchange). That is why you e.g. can buy 0.5 NEO, where NEO on the blockchain is not divisible.

This is also why technically you do not own the BTC as long as they are on the exchange.