The fixed length encoder means more in those segments. X64 decoders are really complex. I've heard 5% of the die. Unlike other parts like the ALU, SSE/AVX, AES

The fixed length encoder means more in those segments. X64 decoders are really complex. I've heard 5% of the die. Unlike other parts like the ALU, SSE/AVX, AES engines, etc., the decoder is almost 100% "hot" all the time. This adds significant energy overhead vs. a simpler decoder. The other problem is that it's hard to decode lots of X64 instructions at once without this energy cost ballooning. Apparently 4X decode is kind of a magic number. Intel cranked it to 6X in Ice Lake and newer with some magic, but beyond 6X is gonna be hard. The Apple M1 has 8X decode, meaning that it can fully saturate 8 execution units for a lot of instruction level parallelism. Nothing in ARM makes it that hard to go higher. If there's a performance advantage to it we will see 12X, 16X, or even higher decode pipelines. Another big win for ARM is looser memory semantics, allowing for more memory write reordering and more efficient cache designs. Overall ARM is just easier to optimize than X64. X86/X64 is only fast because vast amounts of money has been spent optimizing these designs. If the same amount were spent optimizing ARM, we'd be way ahead of where we are now. P.S. The only area where X64 still really shines is its big complex vector units, but these massive vector instruction sets are to some extent a hack to get around the difficulty of decoding X64 instructions. Think of huge SSE/AVX instructions as big macros. I'm sure ARM will get 256-bit and maybe even 512-bit vector extensions at some point, at which point the gap will shrink a lot in this area too. P.P.S. SMT / hyperthreading is a hack to get around the difficulty decoding lots of instruction in parallel. It's a way to keep a big parallel core busier. It comes with its own overhead though, which increases power consumption, and is a security minefield. Are there even any SMT ARM cores? It's just not as big of a win when you can add more decode width easily.