ng_eiface(4) sets the send queue size to the "sysctl net.link.ifqmaxlen".
By default, this is too small for a modern network (only 50 packets).
So set it to 4096 like if_epair(4) does.
The main problem is that some users are unhappy with the default performance of ng_eiface(4).
P/S. Why is "net.link.ifqmaxlen" so low?