You can push the agent to write succinct code by working in small steps, going back and forth. You push the agent in the right direction but also the other way round because it finds a lot of stuff you might have missed, esp. in complex areas. It also oftentimes does quite reasonable things actually.

You definitely can use those things to improve both speed and code quality at once. But then your speedup is while quite noticeable not astronomical of course. Also the work isn't easier (apart from less typing), just different.

I have a Stackoverflow mode for my agents (essentially a tutor) and sometimes I spend time hand coding but heavily guided by an agent. It helps A LOT to make careful tradeoff when succinct and line by line care matters.