When to use slow

[Sove92]

Unfortunately all bosses who it would be most useful against, are immune to it. Slow is most useful on bosses that hit very fast, where the 20% slow is actually noticeable.

It would also be very useful against plumes, to make the cast time a little bit longer, but sorry, that doesn't work. The heart is vulnerable to it, but the heart doesn't perform any attacks of its own, they all come from Titan, who of course, is immune to it, and can't be even be targeted during the heart phase. This also makes any damage cutting debuffs (Virus, Storm's Path etc.) useless against the heart.