Python3, which is itself quite a major rewrite, still leaves the "global interpreter lock" (GIL) problem intact (I may be wrong on this one, I'm not quite certain). Part of the problem is that it's incredibly difficult to solve, and maybe it isn't such a problem if one can work around it.
The GIL issue is that on multi-processor machines, multi-threaded Python programs cannot effectively use the other CPUs/cores because of the lock. The alternative to creating threads is then to create processes instead, but with more overhead costs. In practice, apparently this isn't much of a problem (or so I've read, I don't have first-hand experience with it).
Unless one operates at Google/Yahoo/Microsoft data-center levels, then such performance issues are probably mostly academic. Very informative to think about them nonetheless.
Saidi