Casimir - Yes the plugin is pretty CPU intensive. I'd recommend trying to spawn the object at the beginning of your game while its loading. then make it invisible. This should put it into memory already. Then you when spawn new copies of it later it shouldn't lag as much.
However you should know its probably not a good idea spawn a ton of characters with this plugin. Try to stick to as few as possible. Any if there's any that are small, try and export them as a sprite sheet instead.
As for the resizing of the PNG. I'd recommend replacing the PNG's in your spine file before exporting it from spine with smaller source images, then re export.