There are many concepts of "decentralized" exchange and all have advantages and disadvantages: some use limited master nodes or proof-of-stake instead of proof of work.
If you are considering some exchanges who can give you good time to let them sell bitcoin for you at low prices, this isnt satisfactory and realty. You have to become so strongly minded that you only chose best thing out of different options. Dont comprise on things, this life isnt for comprising. You have chances so opt for that one that is more interesting to you at any level.