I'm consistently seeing personal and shared anecdotes of a 40%-60% speedup on targeted senior work.
As much as I like agents, I am not convinced the human using them can sit back and get lazy quite yet!
That sounds plausible if the senior did lots of simple coding tasks and moves that work to an agent. Then the senior basically has to be a team lead and do code reviews/qa.
Eeeh, I spend less time writing code, but way more time reviewing and correcting it. I'm not sure I come ahead overall, but it does make development less boilerplaty and more high level, which leads to code that otherwise wouldn't have been written.