I just wanted to chime in to support what Neil said. You have a nice hardware setup, but as soon as you see "iPhone/GoPro" footage as your source, you can't help but...
You hardware is only going to take you so far, and no amount of little setting tweaks here or there are going to make as significant of an impact to optimization as the media you work with.
Phone footage is probably the single worst thing you can put into editing software right now. HEVC is going to be the most difficult video codec to decode, mixed with Variable Framerate, which is like poison for editing software. That'll give you media pending issues, longer than usual encode times, frame sub errors and errors of all kinds during playback and export, render glitches, audio desync, etc. Like I said, poison. It won't always have problems, but it's just not a stable kind of media to work with. Variable framerate = variable results.
I don't know the specific specs of the GoPro footage you're working with, but it could be any combo of H264, H265, and sadly I think recent models are even capable of shooting VFR as well.
Point being: Hardware is only half the battle. Media makes a huge difference when it comes to performance and stability. With the right video codecs and workflow somebody with 1/10th the computer you have could probably match the performance and editing experience you get.
Another potentially flawed but hopefully effective analogy. You could buy a Ferrari, but then pour waterered down gas in the tank, and it's probably not going to run very well... It's not the Ferrari's fault. It's capable of great things. But the car - the hardware - is not the full picture of what it takes for it to run at its peak performance.
 
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