Creating/destroying bulk VLAN interfaces takes too long

Hello,

I'm using FreeBSD stable/12. Creating/destroying bulk vlan interfaces takes
too long to finish. Running parallel doesn't matter.
Is there any fast way to create 100 vlan interfaces?

seq 1 100 | /usr/bin/time xargs -t -n 1 -I % ifconfig em1.% create
...
ifconfig em1.99 create
ifconfig em1.100 create
       14.78 real         0.03 user         1.19 sys

with 4 parallel workers:
seq 1 100 | /usr/bin/time xargs -t -P4 -n 1 -I % ifconfig em1.% create
...
ifconfig em1.99 create
ifconfig em1.100 create
       14.46 real         0.03 user         1.20 sys

destroying:
ifconfig -g vlan | /usr/bin/time xargs -t -n 1 -I % ifconfig % destroy
...
ifconfig em1.98 destroy
ifconfig em1.100 destroy
       21.89 real         0.03 user         1.64 sys

Any suggestions?