I've read arguments that CISC now has a speed advantage because its denser instruction encoding is more friendly on the cache. If true, then ARM's RISC instruction set would hold it back from ever competing with x86 on speed.
x86 doesn’t really have a dense instruction encoding. The old un-prefixed scalar instructions are dense, but they do much less work per instruction than SSE or AVX instructions, which have two or three byte prefixes (four with AVX-512). arm64 instructions are all four bytes, for comparison.
The upshot is that simple scalar code tends to be somewhat more compact on x86, and tuned vector code (likely to be found in perf-critical routines that are CPU-bound) tends to be somewhat more compact on arm.
More to the point, loop buffers and µop caches on the last couple generations of processors make encoding density mostly irrelevant to performance (though occasional pathological examples do still exist).
Since ARM11 (ARMv6), cores have been running on Thumb 2 (16-bit instructions) and only jumping into regular 32-bit instructions when necessary. Before that there was the Thumb instruction set which according to [0] was introduced in 1994.
This makes ARM (especially ARMv7 and newer) a mixed setup like x86.
Thumb2 only became standard with ARMv6. There are a few ARMv6 cores which implemented Thumb2 (as opposed to Thumb1), but they aren't in many commonly used products. The ARM1176 for instance as used in the Raspberry Pi is Thumb1 only.
From my experience certain applications, mainly server workloads, have a high instruction footprint. I-cache optimizations are necessary even on an x86 processor. So there might be some weight to this argument. I'd like to see the difference in behavior between the two architectures.
