Entity Movement Speed

How do you change movement speed of an entity? Sound simple enough, maybe i'm just missing something. I'm trying to create a custom potion that in it's effect would incloud slowing down effected entity if that matters.

PS:
Obviously i'm new to mCreator and not trying to do something monumental, but somehow after succesfully doing a lot more complicated sftuff im stuck at this? I connot even begin to express how infuriated I am, 'couse for last 3 hours I couldn't find any answer to that stupidly simple question