Personally haven't use contagion in frontline cause people tend to die quickly. But in other content she did response late, I guess it's may be due to her GCD, still she will did it, so, what should we do in the delay? What I do is when B2MB were up I would hit contagion (considered mobs were not gonna die to quick), run into the mob hit MiasmaII and start spamming Blizzard II then when contagion hit, I wound bane them. Normally it will hit right after MiasmaII hit. I find if the mobs die quick it not worth contagion only Shadow flare-B2MB-bane-spam blizzardII would be enough.