On AMD, threads may as well be cores. If you take a Ryzen and disable SMT, you're basically halving its parallelism, at least for some tasks. On Intel you're just turning off an extra 10-20%.
https://www.techpowerup.com/review/amd-ryzen-9-9700x-perform...
The benchmarks show a 10% drop in "application" performance when SMT is disabled, but an overall 1-3% increase in performance for games.
From a hardware perspective, I can't imagine how it could be physically possible to double performance by enabling SMT.
>From a hardware perspective, I can't imagine how it could be physically possible to double performance by enabling SMT.
It depends on which parts of the processor your code uses. SMT works by duplicating some but not all the components of each core, so a single core can work on multiple independent uops simultaneously. I don't know the specifics, but I can imagine ALU-type code (jumps, calls, movs, etc.) benefits more from SMT than very math-heavy code. That would explain why rustc saw a greater speedup than Cinebench, as compiler code is very twisty with not a lot of math.