OK. I sounds like you may be hardware limited. 4K editing needs plenty of resources, CPU and GPU and RAM (all the acronyms

). On my 2-1/2 year old PC, I have Windows 10 64-bit, an Intel i7 6700 processor, 16 GB RAM, a 500 GB solid-state drive (for operating system and programs only), an nVidia GTX 1070 with 8 GB memory and 22 TB of hard disk space. Many of those things are upgrades that I have added over time.
I assume that you are overlapping the video clips to Crossfade. I use this regularly. It blends image and sound perfectly. You can also try a Fade transition. The Fade transition can be placed at the end of one clip, beginning of the next clip or straddling the intersection line. You get different results with each. I hope you can get this worked out. You get a lot for your money with PowerDirector and it's relatively fast handling 4K video.