No it doesn't slow normal speech development, when you sign you use the verbal word too, it is the same type of signing you do with Makaton that you use with children with Autism, however a child with no developmental delays will communicate verbally faster (so will a child with developmental delays that previously didn't communicate, or has low communication)

Their fine motor skills in their hands develop faster in their hands than their mouths, and so signing can stop the frustration of communication and also encourage them to communicate more because their is a great chance they'll be understood!

If you think about it children already sign by pointing (that is a simple sign!) but this just teaches them if the place their hands in a certain position, they can communicate their needs!

HTH!