iTunes sync suddenly very slow

Hi,

I have an iphone 5 and I use the latest itunes on a windows 8.1 laptop to sync music. Usually it takes my phone about 5 minutes to go from empty to completely filled with music. This morning I tried to sync music as usual to replace a couple albums on the phone with a couple different ones. The phone asked me if I trust this computer or not (it usually doesn't ask on my home laptop) and then started syncing incredibly slowly, one song each 2-5 minutes, so for hundreds of songs it would take many hours. The problem suddenly started this morning. I didn't do anything remotely unusual with the phone recently except yesterday I charged it from a library computer (I pressed "don't trust" that time). But I've done that before many times. Any idea what could be causing this?