I don't think there is much of a major speed difference. You'll still be better off with iron py and other similar branches that have more compiled components. Python 3+ will get all the latest and greatest stuff so it might be faster in a lot of ways, but nothing comparable to the difference between other languages.
You can theoretically use your own python with Houdini, if you wanted to a different library, and then just import the hou module. Generally this is a lot more work to maintain, but bigger studios have a few good reasons to do this depending on their internal library of tools they developed over the years.