Usually, if you want full control over your save/load system, the best way to do it is from scratch or just using Java serialization. There are other options, but I already stated them in this topic (which might be a good read)
It all depends on the control you want. Most of these systems are already pretty fast. Reading from a file is one of the most basic computer commands after all. The speed issue comes from how you plan to organize your data. But to answer your question directly.
Just use a .txt file to write/read data.