You still need to do most of the grunt work, verifying and organizing the code. it's just you're not editing the code directly. Speed of typing out code is hardly the bottle neck.
The bottleneck is visualizing it and then coming up with a way to figure out bugs or add features.
I've tried a bunch of agents, none of them can reasonably conduct a good architectural change in a medium size codebase.
There's a paper that came out in the latter half of last year about this. I wished I kept its name/publisher around, but in synopsis is once you reach a particular amount of complexity in the task you're trying to achieve you'll run out of context to process it, compressing the context still loses enough details that the AI has to reconstitute those details on the next run, again, running out of context.
Currently at least, the task has to have the ability to be broken up in smaller chunks to work properly.