Rendering time is horribly long and not using GPU

Posterize time is a non GPU accelerated effect, hence the long export time with an unfriendly editing codec.

Yeah right that explains the technical side. But what can I do to fix it? Surely my GTX 1050 pc isn't way worse than my MBP? 

That device ... can create some of the truly nastiest long-GOP media made. I've seen people test the clips with apps that sort out the iframes and dependent intermediary frame-sets. There are apparently ways to create "partial iframes" ... so some clips have gone over a hundred frames between complete iframes.

At that point, the system has to decode and decompress two or three iframes, and then ... create from data sets ... most of the pixels of the other hundred and some frames ... before it's ready to play the next ... frame of video.

Yowza.

Video stream: HEVC , bit rate 99.3Mb/s, 3840*2160 (16:9), 119.880 FPS, HEVC

The free utility app MediaInfo is useful for that sort of thing. Install it, then drag/drop a clip onto that applet's desktop icon, it opens ... go to the view that was called the "tree view" ... that might have changed recently. Anyway, it lists all the details of the video and audio in separate sections.

GoPro ... 120fps 4k ... um, there's not much out there that puts anywhere near that much load on a system for playback let alone render/export. 6k ProRes 444 is a ton easier on the system ...

GoPro 10 - I'd assume MP4 and Codec I'm not sure, with bit rate I'm not sure either on how to check?

4k 120fps ... what created it, and what format/codec? Bit rate?

You're right. Apologies for not adding this in OP. In the first specs I mentioned it's got a config of 16GB ram and 120GB nvme ssd (not much) but has 70GB free. Second PC I mentioned (MBP) has 256GB SSD with maybe 100-70GB free, 8GB memory. 

As for the media used, 4k 120FPS footage, 9 minute clip. Is there other info you need?

That's still a long discontinued 8th gen 4-core CPU ... and a 3-gen back GPU also. You don't mention your RAM, which is another potential issue. And how much room and type of drive for cache files.

Plus the media used ...