Why do some GeoPDF documents open slowly in Adobe Reader?

The primary cause of performance problems with GeoPDF documents and all documents that use PDF object data for the storage of data are related to architectural problems with PDF object data itself. That is, it’s not unique to GeoPDF documents, but to PDF. This is not an easy question to answer definitively, but in general, it’s best to keep the number of features and attributes under some tens of thousands, which often is not very much when working with geospatial data sets. This is one of the reasons TerraGo implemented GeoPackage feature attributes.