When you switch from another application back to Final Cut Pro, there may be a delay or you may see the spinning disc cursor for a moment.
This can occur if you have a network volume mounted (e.g., via File Sharing) that is connected via a slow network link.
Unmount any network volumes not required by the work you're doing. Any slow network volumes that are mounted to your computer may cause occasional delays in Final Cut Pro performance, as it polls drives for availability.
See "Final Cut Pro: Choosing a Hard Disk" for additional tips.