An mbuf is a basic unit of memory management in the kernel IPC subsystem. Network packets and socket buffers are stored in mbufs.
Try increase nmbclusters.
Navigate to System → Advanced → System Tunables.
kern.ipc.nmbclusters=65536
or SSH in and type:
sysctl kern.ipc.nmbclusters=65536
NOTE: You should also make the appropriate changes to /boot/loader.conf.local in order for the change to persist across reboots.
Reboot to make this change stick.
The actual value for nmbclusters depends on the the number of network interfaces, CPU cores, and few other factors - a bit of experimenting is required to find the optimal value for each system.