Your 12K source in 8K timeline will be during playback decoded at eg 2K, so this is why it all works fine.
When you start exporting at full debayer and quality an 8K master then check if you have realtime export

It still doesn't change fact that you can do some work even with 12K source, which is great.
It will work similarly well on any laptop, specially one with good GPU. Apple unified memory and new design has its advantages as well.