I had been editing videos using CyberLink's PowerDirector 14, which the only problem I have encountered was rendering thumbnail videos of 4k files.  The video would freeze and stutter while the file rolled merrily on, playing audio perfectly.  If I stopped the playback, it would not stop at the point in the file it was playing.  It was maddening trying to edit 4k video, especially when I was trying to synch it to additional audio tracks and videos.
Yesterday, I got an email from CyberLink offering me an upgrade to PD15 for fifty bucks.  On a lark, I tried it.
So far, it seems to render 4k videos as smooth as silk.
I also got a bunch of other editing software as part of the suite.  Like PhotoDirector 8, which appears to have some nice portrait editing features for skin, hair, teeth and eyes.