This is really, really easy to reproduce. Just get a high level character and go to Dummy and try to cast Spirit Plague over and over as fast as you can. You can do the same with another spell like Rip Blood (also no cooldown) to see the huge pause between casts of Spirit Plague.
I took a look and I do not see any issues on my end.
Could what you are experiencing be the fact you converted Spirit Plague into a combo skill by taking Conductor of Plagues? What you are mechanically doing is activating the skill on the dummy and then making it spread (which does nothing on a single target), then activating it again.
Also, I have noticed that I cannot cast a 2nd Spirit Plague on a different target within that 2 seconds. i.e. it’s still in some cooldown mode, I’ll try to get a video from a real fight.
I was able to reproduce the bug when I target two different monsters here:
Also, a question on Life’s Journey. Is it applying Spirit Plague via my skill tree (assuming it is)? Does this mean the Conductor of Plagues will effectively not work with Life’s Journey? (assume yes by your reply).
Correct. Second cast in the combo skill does nothing but spread, the next after that is a normal cast. You can tell what part of the combo you are on from how the ability looks in your hotkey bar. It will look more green when it’s on the “spread” functionality.