During the 0.5 seconds in which a squid is invulnerable after sustaining damage, the squid will still produce ink particles when attacked by an entity. This can lead to enormous quantities of particles when rapidly tapping a squid, causing a significant drop in FPS.
I expected that the squid would produce ink only after it has been successfully "harmed" (though certain entities may deal no true damage) through another entities' actions against it. However, even if not successful, all entity attacks that can be directed towards squid will produce ink particles.