Then you are a much better developer than me (which you may very well be). I'd like to think I'm pretty good, and I've many times spent hours trying to think through complex SQL queries or getting all the details right in some tricky equation or algorithm. Writing the same code with an AI often takes 2-20 minutes.
If it's faster for me, it might not be faster for everybody, but it is probably faster for many people.
In a sense you're slowly building the LLM in your head, but it's more valuable there because of the much-better idea evaluation, and lack of network/GPU overhead.
Same way as any other code. You look at it, ask the 'author' to explain any parts you don't understand, reason through what it's doing and then test it.
The only reason to bother doing that with a junior developer is to teach them.
I find having an AI write out the algorithm and then walking me through the steps and generating test cases for all the corner case much faster than, looking up the algorithm online, trying to understand it, implementing all the details and then writing a test suite for it by hand. But I guess YMMV.