I'd not install any drivers for any hardware except for what Apple supplies. Apple tweaks those things for their own use. There are the Apple version of drivers, and the generic versions. Which did you install?
Also, what is the source of the video clips, do they happen to be 4K?
How much RAM does you MBP have?
Where are your media and Libraries located?
Try these troubleshooting steps.
1- Go to Preferences and turn OFF "Background Rendering" and never turn it on again.
2- In the Browser select the Library you're working with. Go to the File menu, to "Delete Generated Library Files" and delete all render files.
3- Quit FCPX, relaunch while holding both the Option and Command keys. In the resulting window, chose to delete Preference Files. This will not touch any Library, just remove files FCPX easily regenerates when it launches.