Why does iTunes sync take so long?
Assuming there is nothing wrong with your setup and that there isn't anything causing additional delays to your sync, you might want to consider two things that impact the iTunes sync speed:
- Encoding of audio files as they get loaded on to your iDevice.
- Speed of USB 2.0 interface @ 480Mbit/sec.
The first one would only apply if you are syncing new audio files to your device and you have the option enabled for downsampling AAC/MP3 files. It is convenient if you want to minimize space, but you can tick a box that says any audio file you put on your iDevice will be adjusted down to a (say) 128Kbps bitrate to save space on the iDevice. If you have this, it does the encode single-threaded while you're syncing and thus adds time to the sync but for the obvious benefit of taking up far less space on the iDevice.
The second point is the far more likely culprit since you're probably moving megabytes or gigabytes of data around. Pictures on a recent iPhone take a few MB each, videos you captured can run into the hundreds of MB and naturally DVDs/iTunes movies probably take closer to a gigabyte. If iTunes is deciding to take a backup as it does periodically, that could be the size of your phone (16GB/64GB/128GB/etc.) Now pumping this through a 480Mbit/sec USB 2.0 interface would theoretically get a maximum of 48MB/sec, though USB overhead probably chops 20% off of that so maybe you're getting 40MB/sec so for a backup of 16GB of iDevice data it would take nearly 6 minutes.
Presumably the lightning interface can handle USB 3.0 speeds, but only the iPad Pro as of 2015 apparently has that capability, though I don't think Apple has officially enabled that yet (as of May 2016) so we don't know how fast it could go and I don't think Apple has published the specs. For all your other devices, you're at the speeds mentioned above.
iTunes is a bit of a dog and capping at USB 2.0 speeds isn't going to help things. The best I can suggest is to enable WiFi sync (if available) so that you can at least have your iDevice sync with your computer while not tethered with a Lightning cable.