Who often wonders why Final Cut Pro X suddenly suffers an inexplicable performance slump could find a possible explanation in Google's Chrome Browser. The Twitter user Felipe Baez claims to have discovered that an open Chrome browser under MacOS can make Final Cut Pro X work much slower and even freeze in extreme cases.
The reason for this can probably be found in Apple's VideoToolbox. This is a low-level framework that allows direct access to hardware encoders and decoders. Chrome seems to use it, but that doesn't explain anything. In fact, any program that can play videos is allowed to access these hardware functions via the operating system in order to relieve the CPU of decoding. However, it is conceivable that Chrome will not release access to the hardware again after use or will permanently reserve access exclusively and thus block it.
The solution is therefore simple: Simply close Chrome completely during editing or use another browser. And if there are still occasional problems with Final Cut Pro X, maybe just check out which other programs are still active that could encode or decode videos.