Does your form have either form and/or submission attachments? If so, roughly how many? @ahblake has recently informed me that forms with many media attachments can take unreasonably long times to load so I wonder if you have run into another case of that.
If that's not it, it would be helpful to see the form definition if possible.
yes it has media attachments, up to 10 or more...
you can test the public form I published, hope it helps.
I can also give you access to the test project.
If you keep refresh the page eventually it opens the form.
It doesn't have any attachments, in fact, I tried creating an attached CSV file for a section of the "choices" tab with approximately 900 records, and the loading time for the submission was the same.
I conducted some tests and realized that I have different loading times on different computers. For example, on a CPU i7-7500U with 12 GB of RAM, it takes 9 minutes to open the submission, while on a CPU i7-9700K with 16 GB of RAM, it takes 4 minutes and 40 seconds to open the same submission (both computers with high-speed fiber-optic connections).
I also noticed that the Chrome tab opening the "enketo" has high CPU consumption, which corresponds to the theory that better CPU results in faster loading speed.
Lastly, I created a test form to replace the one in Central, where I removed all the "relevant" sections, and the loading time for the submission dropped to almost one-third of the previous time.
I forgot to mention that all of this happens when I want to edit a submission to clean up the data.