Postgres-XC is not just a fancy version of PostgreSQL but rather a truly distributed system. This means that you cannot just store data and expect things to be fast and efficient out of the box. If you want to optimize for speed, it can be highly beneficial to understand how data is stored behind the scenes and how queries are executed.
Sure, you can just load data and things will work, but if performance is really an issue, you should try to think of how you can use your data. Keep in mind there is no point in using a distributed database system if your database system is not heavily loaded. So, if you are a user of Postgres-XC, we expect your load and your requirements to be high.