How large is your project file? Not with the media. Just the FCP project file.
I’ve had situations where I’ve had a 300mb project file and was it running from my external hard drive. Once I moved the actual project file to my desktop and opened the project from there it ran better.
But, JAAP is right. This could be a number of issues.
How many layers of video?
What’s your resolution?
Is your video native to a video setting? Or not?
RAM?

Stephen Vidano
TuneSociety.com
“Set Your Music Free”