Quick summary

Copying your photos for upload is slow. It seems your photos are in a cloud-synced folder, such as OneDrive, Google Drive, or Dropbox. Your photos need to be on your local hard drive for Imagen to upload them properly. 

How to fix this

1. Copy or move your catalog with its folders to your local hard drive. Try again. If this doesn't solve the upload issue, continue to the next step.
    
2. Create Smart Previews for your photos. Try uploading again. If the upload still has issues, continue to the next step.
    
3. In the Develop module, go to Lightroom Classic > Preferences > Performance and select Use Smart Previews Instead of Originals for Image Editing. Try uploading again. If the upload still has issues, continue to the next step.
    
4. Copy the original photos to a local hard drive, and reconnect the photos to your catalog. If you don’t want to change your catalog, import the photos to a new catalog from your local hard drive, and upload that catalog to Imagen. Contact support if you still need help.