Will sharing slow down my Internet connection?
Yes. Sharing resources does cause slower individual access. However, whether the slowdown is noticeable depends on many factors.
If you share a high-speed broadband internet connection you will rarely notice additional computers browsing, simply because they're not consuming the entire bandwidth, and even if they do, it is often for just a couple of seconds. Also, many web sites serve pages slower than your broadband connection can handle due to their load, internet congestion, etc. You will not notice any slowdown from VoIP calls, or audio streaming either, as they consume very little bandwidth.
You may experience some noticeable slowdown if some of the clients on your network are streaming video, especially High Definition movies, or if you are hosting some type of servers, such as P2P programs, or maxed out game servers. Some of those slowdowns will only be noticeable when your upstream bandwidth is filled up, then it is common for the connection to experience high latency, some packet loss, slowdowns, etc. This can somewhat be mitigated by using more recent routers with intelligent QoS settings that prioritize different types of traffic. Some of those routers also allow for limiting bandwidth per client, so that one PC can't consume all available bandwidth.
If you need helps setting up your NAT router, please check your model in our routers database, and ask questions in our forums.