Nami's passive was recently buffed to 60 movespeed from 40. However, when she uses her speed boost on herself when she has mobis it only gives her 40 movespeed plus the AP scaling instead of 60 plus the AP scaling. Can you fix this please Riot?
You are passing the movespeed soft cap. Once you pass a certain point (I think it's 415) you only get 80% of your bonus MS, and past like 490 it's cut by 50%. I can get the real numbers in a minute.
If you have a problem with getting reduced bonus, pick up a different set of boots. I like Swiftness if I don't need a defensive one and am having problems getting caught out while doing my vision job.
