Your PC or Macbook may have a fast processor and enough RAM but you can still have trouble playing audio / video in iTunes.
For instance, watching movies in iTunes can sometimes get slow and choppy or the frames may freeze momentarily.
Why is iTunes video slow? You can experience slow playback if you are playing video that’s stored on a SATA hard disk or when the CPU is busy or something else.
How to fix choppy playback in iTunes ?
1. Don’t use iTunes – open the video file directly in QuickTime or switch to VLC / Miro.
2. If you are using a SATA hard disk, update the device drivers. If that’s doesn’t fix the issue, disable ReadyBoost in Windows Vista for that drive.
3. Open your Quicktime player preferences and deselect – "Use high quality video setting when available"
4. Open your QuickTime preferences -> Advanced -> Video and select the DirectX mode with DirectDraw and Direct3D.
5. Make sure that you are running just the essential applications on your computer when playing an HD video clip.