This is not my territory, so I may be totally barking up the wrong tree, but… I see your “slow” system is using many SSDs. Are you aware of Wendell’s video and post re slow I/O performance with many NVME SSDs?
https://forum.level1techs.com/t/fixing-slow-nvme-raid-performance-on-epyc/151909
To be sure, the circumstances were different, but as I understood it the root cause had to do with getting so many I/O interrupts so fast that the Linux kernel tripped over its own feet. Wendell says the Linux kernel has since been fixed, but something similar might be affecting your BSD system.
Just a possibility to consider. Good luck.