though not familar with macs....your hardware appears to be good enough to handle that footage. Two things : You MUST be plugged in when editing.....no matter what you do to your power settings, the machine will throttle way back when operating on battery. Secondly : check your power settings again and make sure everything is set for "maximum performance" while plugged in.....I had a problem with my Windows laptop with it choking, even though it was plugged in. I had to manually reset the power settings to all maximum....the default setting on the new machine was throttling the CPU even when plugged in.
i see on NVidia's site that the specs for your GPU are not wholly listed, (i.e. how many CUDA cores,etc.). I have the 765m and it does fine. I see there IS a variant of your card that uses DDR3 memory,but, yours should have the DDR5...that WOULD make a difference if yours was DDR3.
Try overclocking the MEMORY CLOCK on your GPU with the free utility " MSI Afterburner" I safely overclock mine over 900Mhz with no heating problems at all. This yields a 25% improvement in video encoding when the timeline is full of GPU accelerated effects.
Afterburner lets you monitor"GPU usage" so you can see if the GPU is getting saturated,OR, is NOT being used. Use task manager to monitor CPU cores to see if they are getting maxxed out at 100%.....monitor memory,too and disk usage.
Visit the PPBM8...or 7...website to run the benchmark test for video editing. this will reveal any "bottlenecks" in your system. Your laptop has a better CPU than mine and PCIe storage should be VERY fast....allowing everything to go on one drive where PPro usually requires two SEPARATE fast drives. Test THAT for disk transfer speed....the PPBM test includes one.
Make sure your" memory settings " are correct within PPro "preferences" and that unnecessary processes are shut off. Confirm that "Mercury Playback" is active and NOT greyed out, which locks you into "software only" mode for Mercury Playback. If GPU not working properly, make sure that under NVidia settings, that PPro will use the NVidia GPU and NOT the "Iris Graphics". On my laptop, the integrated graphics only runs when there is a small load,then, the NVidia takes over when any larger load comes along automatically.
Be aware that the use of many third party plug-ins,especially ones that do not "thread" well, can slow your machine to a crawl.