Clash Royale buff/nerf calculation

I would like to ask you how you calculate nerf or buff hit speed for the cards in the game. For instance the latest nerf of Rocket Hit Speed for Goblin Machine shows -13%. Which is from 0.286 hits/sec to 0.250 hits/second. So I think if this is nerf, and it should launch its rocket slower, 13% nerf should go in the opposite direction which is 0.32318? Because if it goes lower than the card should launch rockets faster if it is 0.250?

Could someone explain this to me? I have seen this information in every balance change but I am worried that I don’t understand it.

Thank you.