Tak--
In concert with a compression library, human-readable save files might work. It is IO that is slowest, tho decimal-binary conversion still remains a hog.* Xml machine-encoding schemes are a possibility; they are widely used as a compromise between human readability (ASCII), schema conservation (XML), and performance in both disk and CPU.
Here is one written in C.
http://exip.sourceforge.net
The most elegant, though, would be to use the existing parser model to read and write the save file.
You'd have to flatten the various lists--one for dungeon architecture, other for monsters, objects, inventory, and player stats.
As a matter of fact... maybe i should give it a try.
* even in CPU-heavy apps, decimal conversion takes something like 50% of processor time.