I also thought this was bollocks so have just been running pyperformance on a virtualised arm64. I have some figures from Linux running on the same platform. I've not been very scientific about it.
FreeBSD is consistently slower. Between 20-50% across the board. I must admit I'm amazed that Python spends enough time calling into the OS to make a difference this big. Perhaps it's a problem with libc? Maybe there's something going on with memory barriers? My "fist unix" was FreeBSD so I'm amazed and just a bit gutted.
Maybe, it is QEMU/FreeBSD problem, not FreeBSD/Python one?
I'm using FreeBSD for 20+ years on real hardware and never seen huge performance differences for all common software, which is not using CUDA or alike.