I’ve been told that ideall

I’ve been told that ideally you want a hard drive that rotates at 7200rpm. So if you’ve got a 5400rpm hard drive, that may be the issue.

If it is 7200rpm, then try changing the settings in FCP that allow you to edit in lower quality. The footage won’t change quality, it’s just letting you edit in a lower resolution so that your computer can handle the video.

Transferring the video to your main hard drive is the last resort and I don’t recommend it at all. If your hard drive crashes and you lose your footage and project files, you’re out of luck.