Hello Mauricio,
I can’t give you a solution right now, as there is no solution for this at the moment, neither blueprint nor c++. Slow NFT marker loading speed is a know issue of ARToolkit, and there is no solution for it. Loading 30+ markers takes some time. As far as I know the loading time of NFT markers in the upcoming ARToolkit v6 will be way faster. I am going to upgrade once the source code will be available.
Loading markers in a different thread is not supported by the plugin. (I am not quite sure if loading markers in ARToolkit is thread safe or not. )
What you can do at the moment is to use a custom event to get notified if a marker is successfully loaded, so you can update your pre-loader progress bar accordingly.
You need to add some C++ code to the plugin source for this.
I’ll provide you details later today/tomorrow, once I got it working and tested.