I had to pause some of my work getting a current-gen AMD graphics card running on the Pi 5 and testing a 192-core AmpereOne server to quickly post on the M4's efficiency.
I expected M4 to be better than M1/M2 (I haven't personally tested M3), and I hoped it would at least match the previous total-system-power efficiency king, a tiny arm SBC with an RK3588 SoC... but I didn't expect it to jump forward 32%. Efficiency gains on the Arm systems I test typically look like 2-5% year over year.
The chip isn't the fastest at everything, but it's certainly the most efficient CPU I've ever tested. And that scales down to idle power, too—it hovers between 3-4W at idle—which is about the same as a Raspberry Pi.
And the system I bought includes 10 Gigabit Ethernet and 32 GB of RAM; most systems I've used consume 4-6W just _running the 10 GbE controller!
In 1.25U of rack space, you could run three Mac minis, idling around 10W, giving almost a teraflop of CPU performance. (Not to mention there's a fast GPU/NPU, 10 GbE, and tons of high speed Thunderbolt IO in the back.)