Hi there!
It is a bit complex to explain how it works, but I will try.
1 - On the Character Generator scene, the plugin will load all files/pieces that the Generator has. And it will show them on the preview windows. In that scene, the PNG files are loaded each one inside a sprite. So they are layered. One sprite for one PNG file.
2 - After you create the character, the plugin stores the references of all the pieces used. And, on the Map Scene, it will only load those pieces. Not the entire generator. But, different from the Generator Scene, the characters do not have a sprite for each different piece. The plugin creates several sprites, just like the Generator Scene, but it puts them all together inside of a single sprite, creating just one PNG file.
When I add the plugin commands to change character pieces, the process will probably involve redrawing everything with the new pieces inserted/changed with the plugin command.
Now about performance issues, it really depends on the mobile that is running the game and also how many pieces/PNG files you have. There is no way to escape the fact that you will need to load all the png files the character will use. But those files are small. I guess it will only cause trouble if you have like a lot.