What is the difference between multi-core multithreading and a single core?
With all other variables identical and the same computing power per core, 4 cores for 1 second is weaker than 1 core for 4 seconds, for example, because today's mainstream multithreaded search algorithms incur losses, although they widen the search tree. You can think of it this way: in most positions 4 cores for 1 second is not as good as 1 core for 4 seconds, but in some positions it is still better than 1 core for 4 seconds.
Multithreaded search is also nondeterministic: searching the same position at different times is very likely to give different results, a randomness that is hard to reproduce.
Single-threaded search is deterministic. For the same engine, regardless of instruction set or hyper-threading, even on different machines, as long as the settings that can affect the search path are identical (such as hash size, rules settings, multipv) and the hash has been cleared, the 1-thread analysis at each depth is identical; only nps and time differ.