I may be misinterpreting what your saying but this tutorial I think might help:
The only thing is, I 'think' this is just replicating what the Navigation/Navmesh nodes already do for you, so I would be more interested to see what your existing code/workflow is that's 'slow'?
For example if your calling get_simple_path() every delta, your gonna have a bad time and I'm not sure there would ever be any reason to. Also if your using lots of complex collision shapes I've found that tends to slow things down too (a lot).