I own an early 2011 MacBook pro (2.3 I5 with 8gb RAM). With FCP X it performs very well in acquisition and editing (from a miniDV Canon HV30 with firewire) but when it comes to rendering, especially if I apply some grading, the system stucks even on 20" timelines. Until now I tested it only on internal 320gb 5400 rpm drive with only 100gb free space left, and obviously this is the first, and I hope the only, reason. That's why I'm going to buy a second disk. My problem is that thunderbolt disks are still too expensive, usb 2.0 is not an option, firewire is busy with my camera and I would avoid to cascade camera on firewire disk. That's why I'm going to remove superdrive to put an additional 750gb 7200rpm sata disk using a caddy adapter, and use it only for fcp projects. Is it a good solution?
render issue might be from working off the internal HD, always edit off of a external HD (raid preferably). I usually open my project off my desktop and keep all my media on an external HD and work off of that.