Syncing iTunes to phone from external hard drive

Today, I transferred the music that I put into iTunes onto an external hard drive and deleted it off of my PC in order to save space. This worked spectacularly and saved me a ton of space, except now some longer audio files are taking 10+ minutes just load one song. I keep a lot of longer DVD audio commentaries and such on there, but they would usually take a minute or two to transfer, not 15! I guess it's loading from the hard drive to the PC to the iPhone? Would that have an effect? I mean, apparently it does, but why would it be this extreme?