If you just go to the exchange and buy yourself a coin, you can use decentralized exchanges, but if you trade there all day long as traiders do, then you can only use centralized exchanges.
What are you talking about. Being a DEXs trader is not realistic. There everything constantly freezes. And for trading, you need a good speed to respond to market changes.
Look at the Bitshares exchange, there is a high speed of trades, but few traders.