Linux

Yeah but this is Intel, they're not capable of doing something the most efficient way possible. E cores use less power than P cores, but that doesn't mean they're very good at getting the job done using the least amount of power. Currently (using Intel's management) 12th and 13th gen start a task on the P cores, and if it runs for longer than X time it gets shifted over to the E cores where it can churn away. Meteor lake has 3 stages of things since there's P cores, E cores, and LP E cores. If I remember right Meteor lake starts a task on the LP E cores, then shifts it to P cores, then shifts it to the E cores if it's taking too long. But Intel likes to blast the power away with turbo boost and runs the E cores way past their actual efficient zone unless you wrangle them back down. Sometimes it's faster to blast the task away on the P cores then return to idle, other times letting it churn away on the E cores forever is the best way.

Also all those extra cores being active still uses power. So if you went from a 4 core CPU to a 4 + 4 cpu now you have all the same power draw as the old one, plus the extra 4 efficient cores sipping at even more power. I think that's where 12th gen really suffers the most.

Spoilers: they are efficient in space, not really power

So for multicore workloads they are actually power efficient because you get more throughput (more performance iso power)

But for idle that has no benefit

They're not really made for power efficiency, but rather space efficiency. ~4 E-cores fit into the size of a P-core.

They're there to boost multi-core performance without having a huge die-size or increasing latency in the P-cores when doing lightly threaded tasks, essentially.