[jonathanstrange]

That's way worse than loud mastering. There is no reasonable general method of compressing the mixbus at playtime, it depends on the source material and musical considerations. Its not even possible to find the right compression level without using your ears.

To be fair, I believe mastering is one of those domains that may soon be made obsolete with machine learning. But we're not quite there yet, and the existing methods require a lot of DSP power.