Pokebase's page on the Iron Ball just says it cuts speed. How much speed does it cut?
When this item is held by a Pokémon, the holder's Speed is reduced by 50%. If the holder is of the Flying-type or it has the ability; Levitate, it becomes susceptible to Ground-type moves.
This means if a Pokemon's speed is 100 it'll be 50.
Source
Hopefully I've Helped!
Basically, you will be at half your normal speed. Source (scroll down to find the Iron Ball)