In most of Europe thermodynamics are still applicable, so extra heat from an old inefficient CPU still ends up un your house, raising your ambient temperature and making you uncomfortable (in the summer) regardless if you have AC or not.

Lack of AC doesn't make the problem go away, it just makes it a better case for investing in thermally efficient electronics.

But yes - modern lower power hardware (Apple Silicon, lower power AMD Zen, etc) can be drastically faster than older hardware, at significantly lower power - and massively lower total power consumption for work done.

You can get wins here by making sure even modern hardware is running in the more power efficient part it's perf/watt curve - for example, ECO mode on modern AMD hardware (105W+ to 65W, for instance), reducing the board power limit on GPUs when doing compute (~420W to 225W, for most efficient on my 3090!), etc. You lose remarkably little performance by doing so.

At homes what is more common are ductless split systems, combined with an architecture where you have windows that can be opened when the weather is not that hot.

It is completely idiotic to cool a whole McMansion when the 3 or 4 people inside are occupying no more than 30% of the space most of the time.

Where are you getting these numbers from? Typical central air CoP (Coefficient of Performance) even when it's > 100 F outside is at least 2, typically 3-5. Assuming based on current outdoor temp the CoP is 3, it'll remove 100 W of heat with 33 W of energy. So instead of 100 W -> 300-500 W you're looking at 100 W -> 133 W total.