Proxies are your best friend
Here is an example with some 4K test video, my computer is to slow to handle, but with proxies it work without a problem.
The first time you activate proxies, it takes some time because shotcut need to generate 1080x720p edit friendly version of your source video files, but after that is works very fine.
I’m with TimLau on this - proxies are the way to ease the stress on you resources and speed up editing time. I use 960x540 proxies. Just set proxies on in the settings menu, drop some files in the playlist and go get a cup of coffee. When you return, the files in the playlist will indicate that they are proxies.
One thing though, I don’t find that shotcut automatically deletes these files when you’re done with your project, and though smaller, they do add up. In my windows system I have a desktop icon linked to the folder they’re stored in and every once in a while I open it and clean out all the proxies dated before today. You’ll find that if you missed or forgot something, if you put the original file back in the playlist, and the proxie still lives, it will automatically be linked and useable.