Great PC, Poor performance when only basic editing

You did not state your Shotcut version or OS. In any case, please be aware that when you first add a clip to the timeline, it will be decoding the entire file in the background to generate audio levels for the waveform and thumbnails. This slows down the user experience for a little while. My Windows 10 PC can play (i.e. no filters in Source player) a 4K 60 fps H.264 MP4 without dropping frames in Shotcut. It is also known that the Timeline performance is not great when it is very long or has many tracks with clips on most of them most of the time (dense). You should also try the different options in Settings > Display Method, in particular OpenGL:

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