There are two common causes for frequent rendering in FCP:
1) You’re trying to work with H.264 material. You should transcode that to ProRes422 before bringing it into FCP.
2) Your Sequence settings don’t match your footage settings.
When your playback is stuttering, this is usually because your storage device is not transferring data fast enough to your computer. If your footage lives on an external drive, it should NOT be connected through USB2.0. Instead, go with FW800 or eSATA.
Also, if your footage lives on your system drive – the main hard drive of the computer – that will cause performance to drop as well.
