Keeping in mind that lightning networks for altcoins are either very small or non-existent, a decentralized exchange has to find another way to swap coins that don't share the same format. I'm talking about coins that follow formats like ERC-20 and TRC-20 for which there exist services to swap them with a similar format.

Exchanging pairs like BTC <--> ETH or ETH <--> TRX is going to be a problem because you somehow have to commit both trades to their own blockchain at the same time, and ensure that it hasn't committed only one of them.


Generally speaking, since the buyers act as nodes themselves then the speed is usually limited by their internet connections and also by the networks of the crypto currencies that are being traded. If you construct a routing table that has a map to the fastest nodes, organize everything around that and hand it to all your DEX nodes then you can somewhat ameliorate the internet speed problem.
 

You can, but these are going to be traded at your own defined prices because the market value is different for each exchange anyway. Even google and stock websites source their price info from Coinbase or another exchange.