Based on what you've said, I'd say the issue isn't so much your hardware. It's your source footage.
I could be wrong -- but before you go out and spend a grand or two on a new system, consider that maybe the issue is that you're trying to edit 60 fps 4K from a GoPro. Many editors struggle with this type of footage.
If you can, set your camcorder to shoot in 30 fps 4K. That may make all the difference. (You can also convert your footage to 30 fps using a program like Handbrake. But it's nice if you can leave the conversion step out.)
Also, as I said above, try editing using Shadow Files, a proxy system that creates a scaled down version of your video for editing, then applies your edits to the original footage for output.
But using 30 fps 4K rather than 60 fps may alone make all the difference, IMHO.
No need to rush out and spend a fortune on a new comptuer if you don't have to. Especially when it may not fix your issue if the real issue is your source footage.