Avoiding the overhead of python's startup time alone would be nice, hg is impressively fast for being written in Python, but could be much faster and memory-eff

Avoiding the overhead of python's startup time alone would be nice, hg is impressively fast for being written in Python, but could be much faster and memory-efficient in a compiled language where you have control over memory layout. And that is without going into the potential of perhaps taking advantage of concurrency and parallelization for some things.