FCPX runs always with 100% CPU usage

There is a problem with your FCPX installation. A corrupt preference file, cache file, temporary file, or program file is most likely the cause. Do Not use FCPX until this is resolved.

I would consider starting with a program like Yosemite Cache Cleaner 9.0.2.3.0 (in your case look for Mavericks Cache Cleaner.) Use it to clear system and user caches, etc. Also, restart the computer in Safe Mode, then restart it normally.

Navigate to the /Home/Library/Containers/ folder and locate the entry for FCPX, if there is one, and move it to the Trash (don't yet empty) then restart the computer.

Exposing the /Home/Library/ Folder

Pick one of the following methods:

A. This method will make the folder visible permanently. Open the Terminal application in your Utilities folder and paste the following at the command prompt:

chflags nohidden ~/Library

Press RETURN.

B. Click on the Desktop, press the OPTION (⌥) button, select Library from the Finder's Go menu.

C. Select Go To Folder from the Finder's Go menu. Paste the following in the path field:

~/Library

Press the Go button.

Since the problem seems to occur with FCP X specifically, start by deleting the application's preferences: hold down the option and command keys while starting the application and confirm when asked. Deleting preferences is usually the first troubleshooting step, since corrupt preferences are known to cause inumerous problems. Note that this does not affect your libraries in any way.

There is also the possibility that some of your libraries or media is corrupt.

So, as a second step, try starting the application without opening any of the existing libraries. Do so by holding down the option key while starting and then choose New...

If the problem persists, it may be time do a complete reinstall of FCP X. Use FCS Remover to completely remove the software and its auxiliary files.

Then download a fresh copy from your purchases section of the App Store.

Then I would simply remove it and reinstall it. If FCPX comes with an uninstaller then use that. If not then see the following:

Uninstalling Software: The Basics

Most OS X applications are completely self-contained "packages" that can be uninstalled by simply dragging the application to the Trash. Applications may create preference files that are stored in the /Home/Library/Preferences/ folder. Although they do nothing once you delete the associated application, they do take up some disk space. If you want you can look for them in the above location and delete them, too.

Some applications may install an uninstaller program that can be used to remove the application. In some cases the uninstaller may be part of the application's installer, and is invoked by clicking on a Customize button that will appear during the install process.

Some applications may install components in the /Home/Library/Applications Support/ folder. You can also check there to see if the application has created a folder. You can also delete the folder that's in the Applications Support folder. Again, they don't do anything but take up disk space once the application is trashed.

Some applications may install a startupitem or a Log In item. Startupitems are usually installed in the /Library/StartupItems/ folder and less often in the /Home/Library/StartupItems/ folder. Log In Items are set in the Accounts preferences. Open System Preferences, click on the Accounts icon, then click on the LogIn Items tab. Locate the item in the list for the application you want to remove and click on the "-" button to delete it from the list.

Some software use startup daemons or agents that are a new feature of the OS. Look for them in /Library/LaunchAgents/ and /Library/LaunchDaemons/ or in /Home/Library/LaunchAgents/.

If an application installs any other files the best way to track them down is to do a Finder search using the application name or the developer name as the search term. Unfortunately Spotlight will not look in certain folders by default. You can modify Spotlight's behavior or use a third-party search utility, EasyFind, instead.

Some applications install a receipt in the /Library/Receipts/ folder. Usually with the same name as the program or the developer. The item generally has a ".pkg" extension. Be sure you also delete this item as some programs use it to determine if it's already installed.

There are many utilities that can uninstall applications. Here is a selection:

1. AppZapper

2. AppDelete

3. Automaton

4. Hazel

5. AppCleaner

6. CleanApp

7. iTrash

8. Amnesia

9. Uninstaller

10. Spring Cleaning

For more information visit The XLab FAQs and read the FAQ on removing software.

I strongly advise against these blanket "uninstallers" for FCP X.

Like I wrote in a previous message, use FCS Remover. It is free and is made specifically for uninstalling Apple video applications, including FCP X, by a company that knows about video software.