Saving game data with performance in mind

I am working on small MMO project where data loading and saving speed is very important. What is the best choice between:

  • performance (most important, no more than 0,2 s for 10 mb of data)
  • compatibility (possibility to add/remove fields without compatibility loss)

Currently I am using KryoNet for both networking and saving server state, but I am wondering if there is even better solution.