Copy-on-write means twiddling with the MMU, and TLB updates across cores ("TLB shootdowns") can be very expensive. If the process is not threaded, then the OS

Copy-on-write means twiddling with the MMU, and TLB updates across cores ("TLB shootdowns") can be very expensive. If the process is not threaded, then the OS could make sure to schedule the child and parent on the same CPU to avoid needing TLB shootdowns, but if it's threaded, forget about it.