Slow Pathfinding for Larger Matrices

I'm a user of the "pathfinding" Python module. It has greatly benefited my projects.
I've noticed the grid-to-string conversion function can be slow in some cases, especially with larger matrices like my 750x650 grid. On my laptop, it took around 29.89 seconds. I'm keen on optimizing this process. 
(The algorithm i'm using is bi-directional a*, which takes 8.54 seconds)