Is mp3 transfer for Windows iTunes / iPhone 4s slow?
Speed is not an issue between iTunes and an iPhone. You can copy gigabytes of data (music, film,...) within minutes, just like with a standard USB-to-desktop connection on Windows or Mac.

Major advantage: the synchronisation makes it even faster if you only change a part of your music: it only copies the ones that you added for the first time: no more duplicates!