As the other answers suggest, the question isn't "Why doesn't it work well?" Rather, it's "Why does it ever work?" Just be happy that you're able to run iTunes and play MP3s on that mac at all.

For best performance, turn off VM, disable unnecessary extensions, and otherwise turn off anything that steals CPU cycles. Beyond that, you've done all that can be done with a stock model. The performance that you get is what you get.