Slows and enemy speed boosts

[konfeta]

Anyone know if they are additive or multiplicative?

For example, if fast affix (random number) speeds up a monster by 50%, does a 60% slow reduce the monster to 90% (150%-60%) base MS or to to 60% base MS (100 * .4 * 1.5)?

I am starting to get the feeling they are additive...

[Hitori M]

They seem to be additive, but don't take my word on it - please.