It looks like MARKERCLUSTERERSCRIPT / "markerclustererplus@4.0.1" exists in about 4 files, this is referencing an old version of the mapclusterer. Please look into using 5.1.3 - it seems to have received a few performance updates, but I can't say whether there are any breaking changes aside that.
Thank you for notifying us.
We'll have to investigate if references to the markerclusterer script can be updated in a future version of TMS FNC Maps.