[tomhschmidt]

Great questions!

First question is about decentralization. What is off-chain relay in this case? Is that a centralized part of the system? Are there centralized parts of the system?

Off-chain relay is how parties find each other facilitate a trade. As a maker, I can create an order off-chain, but I still need to find a relevant taker who is interested in filling the order by sending it to the 0x smart contracts on-chain. Relayers pool together these off-chain orders and serve them up through a website, so that it's easy to find people to trade with. Note that this process is optional -- you can still trade peer-to-peer if I send you the order over email, SMS, etc. While relaying is centralized, relayers are non-custodial, so your funds are never at risk. We also have plans to make some parts of relaying less centralized in the future through 0x Mesh: https://blog.0xproject.com/0x-roadmap-2019-part-3-networked-...

Can this be used to implement more robust margin trading?

As for margin trading, yes, you can trade dY/dX tokens on 0x relayers, or combine 0x orders with any on-chain lending protocol such as Dharma or MakerDAO to get leverage.

What settlement speeds can be expected?

All trades currently settle on-chain, so the Ethereum block time is our bottleneck. Look for some updates on this near future :)

[Legogris]

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.