You can go faster once you understand the domain reasonably well that you could have written it yourself. This allows you to write better designs, and steer LLMs in the right direction.

"Vibe coding" though is moving an ever growing pile of nonunderstanding and complexity in front of you, until you get stuck. (But it does work until you've amassed a big enough pile, so it's good for smaller tasks - and then suddenly extremely frustrating once you reach that threshold)

Can you go 10x? Depends. I haven't tried any really large project yet, but I can compress fairly large things that would've taken a week or two pre-LLM into a single lazy Sunday.

For larger projects, it's definitely useful for some tasks. ("Ingest the last 10k commits, tell me which ones are most likely to have broken this particular feature") - the trick is finding tasks where the win from the right answer is large, and the loss from the wrong one is small. It's more like running algorithmic trading on a decent edge than it is like coding :)

It definitely struggles to do successfully do fully agentic work on very large code bases. But... I've also not tried too much in that space yet, so take that with a grain of salt.