To expand on this - a middle ground between the centralized exchanges dominating today and the (relatively) slow and expensive ones on the Ethereum mainnet would be a sidechain using a different physical topology and consensus model.

Some examples that have been gaining buzz recently (these are just examples, there are alternatives to each) if you want to read up and grok the statr of implementation and thoughts today: Plasma chains, PoA Networks xDAI, SKALE, PegaSys.