Technically, HTML5 offers the defer and aync attributes to optimize script loading, and it should be possible to add them as options to the plugin config. The “issue” is that this is a plugin.
By that I mean that it a) runs in very different settings (blogs, other plugins etc.) and b) includes multiple lightbox scripts that may or may not work well with to those attributes. So using these attributes should be optional and be decided by the user, not be a default setting.
It would need a developer to implement and (more importantly) test this, but it should not be hard to implement.