BMPC 6K Pro export choppy playback

Hi community 
I've been editing with 6K braw for a few years. I now have a 30 minute nature doc for my final Uni assignment. A H264 1920 plays back smoothly but when I match source it does not play back smoothly in various sizes/bit rates. The smaller 8 GB version is the best but still slightly jittery
the pro res Quicktime took 12 hours nd was 357 GB... so not even practical at all since I need to WE transfer

I have some 4K footage mixed in and some photos. This usually isn't a problem. My laptop is an Omen but older now i5 core and almost out of memory. I think this is my main issue

I'm wondering if it might playback smoothy on another machine. Any ideas? Exporting 4K would be choice but then I get the black bars on the side. 

Thanks for any suggestions. 
Bethan