Unable to work with HEVC, 0fps in Viewer

I shot some footage with my iPhone using the "High Efficiency" setting. I realized that I couldn't edit these without the HEVC Codec from the Microsoft Store so I bought it.

The video files are 4K UHD 60FPS, which I have edited before without a problem when shot on a DSLR.

But when I try to preview a clip down from the timeline, it does not matter which setting I have for the playback quality. My CPU fans will rev up, either it will just freeze on a frame or it will display about one frame every three seconds on average in the viewer windows and the sounds are playing fine. When I pause, it takes a second or two where the program becomes unresponsive before it has paused again.

My computer specs are as follows: i7 10700K, 32GB RAM, RTX 2070 Super, M.2 SSD

I'm using HitFilm Pro 16

Thank you in advance!