With the new KMS updates

Yeah. A delay that's always there is included in casting time. If it only blocks repeated use of the skill, but not other actions i.e. movement or other skills, then it's not part of the actual casting time, and therefore spamming it is slower than the casting time (which is the important thing).

And - if you tank hits (which seems to be not advisable, but still), Cannon has no interruption when being hit. Neither does Triple Fire, if you're still stuck with that.