Effects of slipperiness in OpenAI FrozenLake Environment
According the documentation in that repo:

However, the ice is slippery, so you won't always move in the direction you intend.

That implies is_slippery increases the noise. Specifically, the transition matrix will be effected. It makes sense that your model takes longer to learn when there is an increase in noise between intended and actual action.
There are appears to be many hard-coded numbers in that example. It appears a person with deep knowledge of that specific environment added constants to account for the increase in noise.