Remember, if you have a speech impediment there is somebody you can speak to but it will take ages. If you're offended by this joke, sorry.
People with speech impediments can't help the way they talk and shouldn't be made fun of.
It's just a joke, if you're offended that's your problem.
You probably don't mean any harm with your statement, but what if one of your loved ones has a speech impediment? If they heard you make that joke, they would be really offended by your comments.