If Shotcut is not using 100% of your CPU, then you might be I/O bound. What kind of hard drive do you have? I saw my performance improve drastically when I switched to an SSD.
What kind of files are you trying to edit (file type, codec and bitrate)? Some types of files are more challenging to decode than others.
I can’t tell from your screenshot, but I do not think your computer could handle 4K if you are editing that.
Is the performance poor when playing a file with no filters? Or, can you identify a specific filter that seems to cause the most problems?
Have you tried changing the display method? (Settings->Display Method)?
If your files are interlaced, try changing Settings->Deinterlacer to “one field”.
Changing Settings->Interpolation to “nearest neighbor” can improve real-time response (at the cost of preview quality).
None of the player settings affect export quality, by the way. So feel free to experiment.