If you experience a bug while loading , you should file KML edited in any text editor. Should remove some part lines. Display accuracy will decrease, but in general will be an acceptable image for visualization.
In order to get performance of maps (in any system) the resolution needs to be adequate, too much and it will long time to render and too low then it will not be readable. However there are lot things to consider. please have a look at Johan Persson's blog post on how to simplify map data:
Fyi, IdevioMaps contains already tuned areas for most most places, and also includes tools to simplify user supplied geodata.
Patric Nordström, Idevio