Final Cut Pro Slow

For a start try deleting the preferences.

Hold down the ⌥ and ⌘ keys as you launch FCP X and select Delete Preferences in the window that opens.

If this doesn't help you will need to provide more details of your project properties, footage and computer together with an etrecheck report.

Sudden slow performance could be due to a corrupt video file.

Try re-importing the footage and just add one clip at a time to the project to check whether any particular clip is causing the slowdown.

Etrecheck is free and perfectly safe.

Download it here and follow the instructions.

https://etrecheck.com/en/index.html