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Are you certain that the output from gopro studio is h264 and not cineform codec.Archive/edit output will put cineform into MP4 or MOV container formats.Did you use custom and select h264 instead.Cineform not supported and would explain message.
The reason for the performance in studio is that it uses a proper intermediate editing format(cineform) which is a lot less compressed and studio is highly optimised for it.So edit/playback in it is a lot easier on your pc.When you output to h264 your changing from that to a highly compressed distribution and playback format.You need a lot more power to try and use brute force to edit it.
Ive got an i7 4 core/8 threads /16GB ram and i would change to using a shadow file for 2.7k Especially when you start adding filters and effects which put even more pressure on your pc.Theres nothing wrong with your pc but at that very high resolution a shadow file woud be better to edit a playback format.