Is custom coding faster than Rules when reacting on user creation?
I wouldn't focus too much on speed in situations like this because it's extensibility you will care about as the site grows in function. While your custom code is probably "faster" it's not the measure you should be deciding by since it'll be nanoseconds you're talking about and any good caching strategy will nullify most of that concern.

But to answer your question, your code would likely be faster especially if you're not defining as many hooks as the Rules module.

However, I'd advise using rules for the following reasons.


Rules are exportable
Rules are integrated with all the "big" modules out there.
Never program something you can borrow. :)


Hope this helps.