If you do not need specific namespaces, remove them. Most of the time it is not a big deal in small projects but specially on ASP.NET websites projects it can make a difference. The main difference is that  the compilation time can be significantly reduced and it will speed up your IDE.

If the website contains a lot of namespaces and if the project has C# 3.0 + which enables the use of extension methods it can make a significant difference. By having a lot of namespaces and using extension methods it can make the autocomplete list huge!

One last thing to note that you wont see any particular speed up on the actual execution time of the project if you remove them, they will not affect performance at all once the application is compiled. Its just one of those practices that makes development a bit faster and nicer specially in old computers like the ones I use at work.