Could it be that it accesses the path defined in the configuration element "import_folder"?
The first time I used it I used the "import Files..." dialogue, and imported files from an NFS mounted partition, which seems to have got that value set in the config.xml, and since then I have used the drag'n'drop functionality, but looking at the config.xml it seems the value wasn't changed.
I just created a new project and used the "import Files..." dialogue to set the location stored in the config to a local path, and now openshot starts fast again.
Assuming this is the root cause of the behaviour, suggested solutions:
1. Don't access the path defined in import_folder until "Import Files..." is clicked
2. Store the path from which a file was imported in the import_folder element even if it's added by drag'n'drop.