I use the Voice over recording in FCPX a lot but have never before suspected it to record corrupted files. But now looking back , when editing the sound-clips from voice recorder on timeline, I believe it have always been a little bit slow in response. But before what happened today I did not connect it to just "Voice Recorder". A library where I just made a Voice Recording started to behave very slow in response. After a restart it took 2 min to start up that Library again. Big panic, was the hole Library going to crash ? I made a copy of the Library and started to trash different parts and doing several restarts until i found out it as the Voice Recorded sound file that caused the problem. I trashed it in original library and is now back to normal. So be aware of a possible reason if getting the same problem.
I just made a deeper check and its not Voice over that is the problem. I had put a filter on from Izotope "Nectar 3 elements" thats causing all the problems. Earlier lagging must have other causes. Usually don't put filters on when just cleaning up a Voice Over.