[StillBored]

<i>ARM has never enjoyed a Op/J advantage over x86.</i>

I'm not sure that exactly accurate, its more accurate to say, that there was a lack of market crossover that allowed similar power or perf envelopes.

That is because ARM did/does make much more efficient CPU's, they just aren't anywhere close to the perf of common x86 cores. AKA a low clocked in order ARM with small caches, etc is more efficient per op but it can't touch even a medium size x86. Intel sort of was in that market for a bit and their cores were efficient too, but the main selling point for an architecture is the software around it, and a 50 mhz in-order x86 can't exactly run modern windows in a reasonable way.

Now that ARM & friends are building higher perf parts, the power efficiency keeps getting worse. When someone makes a 5Ghz ARM core it will likely consume more than a couple mW.

The perf/power ratios have more to do with culture and market than ISA.