Our bug tracker at work is an old and heavily customized version of Mantis.
It's heavy in terms of what's loaded for each page view - especially when editing/updating a bug report.
The bug update page takes over 30 seconds to load in Orion 1.0.7. (I didn't use a stopwatch - I counted slowly.) In Google Chrome it took 3-5 seconds before I could edit a field. I downgraded to 1.0.6 and the page loaded in under 3 seconds (faster than Chrome).
https://mantis.local/bug_update_page.php?bug_id=1283003
Quick page load.
Version 1.0.7 (146), WebKit 624.1.7 / macOS Tahoe 26.5 build 25F71
Tahoe (26)