Hi Kristie,
Thank you for choosing PRO and welcome to the support forums 🙂
I see you checked “Yes” to both reverting to the default WP theme and deactivating all other plugins as outlined in our Testing for conflicts guide, but can please confirm this? I did a quick check on a site I’ve got with over 400 venues and didn’t experience any speed/load issues on the venue dropdown when adding/editing an event.
I understand it’s only about one-third of the venues you have, but I just want to confirm you’re not working with a theme and/or plugin conflict. Additionally, we do have a KB article relating to the issue you’re experiencing. Take a look and let me know your findings from the Testing for conflicts guide. Also, send over your System Information as a private reply.
Thank you again and have a good rest of your day!