I may be out of date or wrong, but I recall when the M1 came out there was some claims that x86 could never catch up, because there is an instruction decoding bottleneck (instructions are all variable size), which the M1 does not have, or can do in parallel. Because of that bottleneck x86 needs to use other tricks to get speed and those run hot.

ARM instructions are fixed size, while x86 are variable. This makes a wide decoder fairly trivial for ARM, while it is complex and difficult for x86.

However, this doesn't really hold up as the cause for the difference. The Zen4/5 chips, for example, source the vast majority of their instructions out of their uOp trace cache, where the instructions have already been decoded. This also saves power - even on ARM, decoders take power.

People have been trying to figure out the "secret sauce" since the M chips have been introduced. In my opinion, it's a combination of:

1) The apple engineers did a superb job creating a well balanced architecture

2) Being close to their memory subsystem with lots of bandwidth and deep buffers so they can use it is great. For example, my old M2 Pro macbook has more than twice the memory bandwidth than the current best desktop CPU, the zen5 9950x. That's absurd, but here we are...

3) AMD and Intel heavily bias on the costly side of the watts vs performance curve. Even the compact zen cores are optimized more for area than wattage. I'm curious what a true low power zen core (akin to the apple e cores) would do.

They can always catch up, it may just take a while. x86's variable size instructions have performance advantages because they fit in cache better.

ARM has better /security/ though - not only does it have more modern features but eg variable length instructions also mean you can reinterpret them by jumping into the middle of one.

No one ever said that. The M1 was not the fastest laptop when it was introduced. It was a nice balance of speed/battery life/heat