Forgive me if I've misread, but it seems like you're running 3 VMs with 2 vCPUs each on a dual-core processor. ZFS itself is also processor and RAM heavy (on every OS I've used it on, never have used it on NetBSD).
So, it seems to me like you've taken an already not particularly fast processor and applied a really heavy load to it, especially if you're doing any zRAID or anything else. Someone actually familiar with the NetBSD kernel can correct me on this, but I'm fairly sure that the kernel itself is multithreaded, and handles interrupts on multiple cores. I would guess that with the single vCPU VMs, the processor was able to keep up, but now you've doubled down on the workload, and considerably increased the number of context switches that have to happen. Or maybe I'm completely wrong on every point. I'd certainly take a look at your CPU usage though.
