How did you ensure those threads would actually execute concurrently?

If any operations were GIL-bound (which is extremely common, even if not intentional, by reliance on bytecode instructions dealing with CPython API under the hood, like attribute lookups or iteration special methods), then execution is probably interleaved serially and constrained by the interpreter’s GIL allocation, and slower than just summing serially.

I’ve seen a lot of people who are cocksure they know some contrarian “actually you can use threads” trivia about Python and just naively use the threading module or naively use the newish ThreadPoolExecutor stuff not realizing that no, in fact, it’s not somehow magically always GIL-avoiding to do so.