I recently built a piece of code which downloads a bulk set of data, indexes it for search, and then serves a pretty web UI on top of this with the help of some AIs. Normally I would have reached for sstables, sqlite, etc. This time, because the lookup patterns actually would not have been too efficient on sstables and SQLite would have been overkill, I had an agent take the data structures, pack the text effectively, and build a prefix tree for fast auto completion from the search bar. It was great. I could have done this all before but I wouldn't have. I would have felt sqlite was fast enough. The resulting web server is significantly faster feeling (because the optimized lookup speeds) than an sqlite implementation would feel like.

I think engineers building very complex systems now have a lot of performance knobs to twiddle that would have just been too costly for human effort. Since we constrain the responsibilities of the agent slop is less of a problem. We relegate it to defined tasks with clear API boundaries and test harnesses.