Depending on your MacBook Air model, ProRes is much easier on the CPU than modern H264/H265 codecs, as every frame is stored in a ProRes video file compared to only a couple of frames per second in H264/H265 encoded video (they store so-called keyframes and then store the changes between those keyframes, thus the CPU has more to do than just display a frame, it has to decode/calculate it).
And H264 and H265 are modern codecs used in smartphones and consumer cameras, streaming services, BluRays and so on.
Thanks for the explanation. So the ProRes is likely to be a bigger file but all the video is already in the file, whilst the h.264/h.265 is a more compact file but the CPU calculating how to draw frames which uses CPU power. I think i've got it!