Thanks for sharing those details, and I’m sorry to hear you had to roll back after that frustrating experience.
If you're open to giving InDesign 2025 another shot later, I'd recommend testing it in a new local admin user account and also in macOS Safe Mode. That can help identify whether background processes or user-level preferences are causing the slowdown. Since you've uninstalled for now, no worries, but if you do reinstall, we'd love to help troubleshoot more closely.
Also, just to double-check, was the issue happening with only that 36-page magazine file or with other documents as well? Moreover, please confirm the exact version of InDesign 2025 like 20.3, 20.3.1 or the latest 20.4.
Looking forward to your update.
Abhishek