NOTE: this issue is specific to Windows.
There are situations where a CPU will go to 100% usage when a viewer is running. This appears to occur most in computer with nVidia video cards - but not only.
Another effect that seems to happen with nVidia cards is that the viewer will freeze, seemingly at random, and for varying periods of time. Or it will lock up when you attempt to open profiles or try to search.
There are two solutions possible.
Now try to start Phoenix or Firestorm and see if the situation has improved for you. If for any reason you wish to revert, then quit Phoenix or Firestorm and simply delete the profile - or reenable “Threaded Optimization”.
Should the above option not be available to you, or should it not solve the problem, then you can try to set the process affinity to a specific CPU core:
You will need to experiment at this point; there is no guide as to which core will work better for you. You may also be able to select more than one.
Also, the downside of this is that it will have to be done every time you run the viewer.
There is at least one anti-virus program which will scan every file placed on your computer, even if created by an authorized program. This affects Phoenix, Firestorm (and any viewer really) in that it will scan each and every texture placed in cache, slowing down performance of the viewer significantly, and keeping CPU usage high.
If you find yourself in this situation, see if you can exclude the viewer cache folder from being scanned. Ideally, you should exclude the entire settings folder as well.
You can determine the current location of your cache by looking at Preferences → Network & Folders → Disk Cache Location and Sound Cache Location (if this is set).