Ships that don't have thrust modules on them seem to behave weird. They do not always fire or it just takes them a long time to target you.
I put a turret on each a ship without thrust, and with that it appeared to be fixed, all spawned ships instantly fired their weapons (and so did their turrets).
If they have any thrust modules they act fine and shoot you as soon as possible.
How to reproduce:
# I included 2 blueprints, 1 has thrust, the other has not: {F616}
# mass spawn 20 ships of each blueprint and compare the results while you are sitting in a (god mode) ship
# the group with thrust should all fire their missiles at you within 1/2 seconds
# the group without thrust will fire their missiles only a few at a time and it will take longer till all of them have fired.