You should try both XCP and NXT AE to see the difference in speed. XCP developers also had an issue with the Bitcoin developers after the latter changed something in the Bitcoin protocol which prevented XCP developers from doing some features or somehow limited their options, not sure about the details, but the point is, any DEX riding on top of Bitcoin network will always depend on good will of Bitcoin developers, and that doesn't inspire confidence in investors' minds. Oh, and did I mention that order execution is slow?

Bitcoin does only one function.
XCP does another function.

NXT does both of those + 2 more, and adding a few more in the nearest future.