There isn’t any work in progress so far. I don’t have a release target for 2.1 yet. Still making sure 2.0.1 is nice and stable before starting the next round of features.
I think that CSV import will be able to be made considerably faster than XML. It also has the advantage of being more readable (IMHO). The reason that CSV can be better optimized is that the way liquibase works is to parse the entire changelog into a single in-memory representation which is then sent to the changelog executor. If all the data is in changesets, it all needs to be included in the in-memory changelog. With CSV files, all that is in the changeLog itself is a reference to the .csv file which is read and inserted at run time. It is that reading and inserting of the CSV file which can be optimized.
That being said, it would be worth running liquibase and your changelog through a profiler such as the one that comes with netbeans. I have found that performance problems are rarely from where you expect. There could be other parts of the liquibase code that are causing the bulk of the problem in your case. Without profiling, you really don’t know for sure.