slow wears of after 3 turns or if you counter it with haste.
So if that is not the case for you, then either you are doing something wrong or you encountered a bug. I am more inclined to believe that it is something on your part. The amount of information given by you is rather limited so it is rather difficult to judge.
And as LeBurns said: encumbered is not the same as slowed. Encumbered means you are carrying too much.
With kind regards,