I was experiencing it even with smaller images, even then with a single small image in the thread it would load fast some times and not others which is odd. This also happened to me on a different host when I ran an older version of threads.
Donate to UBBDev today to help aid in Operational, Server and Script Maintenance, and Development costs.
Please also see our parent organization VNC Web Services if you're in the need of a new UBB.threads Install or Upgrade, Site/Server Migrations, or Security and Coding Services.